Set after the end of the first movie, I don't have much experience in writing smut so no flames please. Hope you enjoy.

It may have been the middle of the night, but the BRPD was, as normal, as noisy and active as the London underground during rush hour. Making her way to Hellboy's room, Liz wandered through the organised chaos seemingly oblivious to it all, her mind a jumble of joy and nerves.
For years she had been convinced that being normal was the only way to be happy and so had ignored any feelings she had for the demon, pushing him away whenever he tried to voice his own feelings for her, knowing it hurt them both. But it was only when she'd heard his voice, calling her back from the darkness, back in Rasputin's mausoleum, did she realise that she'd been wrong. She only needed him.

Reaching HBs rooms Liz was not surprised to find agent Myer's standing guard outside. Like her he seemed to have showered and changed into a fresh set of clothes, though it couldn't hide how exhausted he looked. "You never stop working do you John", she asked with a kind smile.
"I'm being relieved soon" Myers replied, trying to stifle a yawn "Can't wait to get some sleep, we all need it, even Reds worn out". Turning he began to unlock the gigantic steel door that blocked her path, pulling it back before gesturing that she should enter, "I'll see you in the morning Liz"
"See you".

The lights inside HBs rooms were off, the only light emitting from the muted TVs that covered the far wall. Surrounding her Liz could see the eyes of his many cats reflecting the TVs glare giving them a rather eerie appearance as they moved about. On the floor, piles of screwed up paper lay scattered everywhere, the failed attempts of letters he'd tried to write to her before. The tattered clothes that he'd worn for the mission were crumpled in a heap next to the giant bed that held the sleeping red mass that was Hellboy. His quiet snores accompanied by the cat's gentle meows.
The demon lay on his back wearing nothing but his black leather trousers, his stone hand and tail dangling off the edge twitching occasionally in his sleep. His horns remained unfiled and harshly broken. She wondered if it had hurt when he'd broken them.

Before she knew it Liz was standing beside the bed, her heart pounding rapidly in her chest as she struggled to figure out what she was going to do next. Being a freak had made her life complicated in so many ways, the fear that you could potentially explode at any moment and destroy everything around you pretty much made making any kind of relationship exceedingly difficult and a physical relationship had seemed completely impossible. The only person who hadn't ever been afraid of her flammable tendencies was Hellboy, though being fireproof was probably part of it. HB had faced his own share of problems over the years, not being able to go out in public and spending his entire live being told that he was a monster, making him believe that he would always be alone.
Watching him lying there, seeming so relaxed, made her ache to reach out and touch him, to trace the many scars that covered his skin, remnants from past battles. She wanted to show him how much she felt for him, to feel every part of him.
Without even thinking, Liz took another step forward, one arm reaching out to gently touch his face when her foot suddenly caught on something solid and furry, sending her sprawling to the floor with a thump whilst the calamitous feline shot under the bed yowling loudly and hissing in resentment.

Inwardly cursing herself Liz started to get to her feet. A quiet deep voice making her jump "Liz?"
Looking up she found herself staring straight into the bright yellow eyes of HB who was now propped up on one arm, looking down at her with a confused expression. "Hi" She mumbled back, accepting his outstretched hand as he helped her to her feet adjusting his position until he was sitting upright with her standing between his legs. Their faces were so close that she could feel his breath on her face, if she leaned forward even a fraction she would feel his lips on hers, something that she desperately wanted. But the nerves were getting stronger, the fear of the unknown making her feel sick. "You al'right?" he asked.
"Yeah just need to keep an eye out for cats", she gestured around the room, "You do have a few"
They laughed for a moment about the silliness of the situation, laughing was good, it temporarily distracted her from the butterflies growing in her stomach. But the second the laughter died down they came right back.

"Red" she whispered as he leaned in to kiss her "I… I can't". Pulling back HB tried to look her in the eye but she kept her gaze fixed on the ground. "What do you mean?" He asked, gently tilting her head up to look at him. It took a moment before she spoke, trying to find the words to explain how she felt.
"I'm… I'm scared Red".
A look of hurt instantly crossed his face as he looked away from her, the arms that had been wrapped around her thin frame immediately dropped into his lap. It reminded her of the look he'd given her the last time they'd spoken in Professor Brooms study, when he'd told her that he'd never give up on her. The same conversation when he'd said he would change what he was if he could. Of course, he would jump to the conclusion that it was him she was afraid of.
"Your scared of me" HB muttered, so quietly she wouldn't have heard it if she wasn't standing so close, "Should've known this was too good to be true"

"Oh god Red no!" Liz gasped, grasping his face in both hands, desperately trying to get him to look at her, which was difficult as he was much stronger than her. "I'm not afraid of you, please don't think that"
"Yeah yeah" he mumbled dismissively still avoiding her gaze.
"I mean it HB" she said, closing her eyes, silently praying he'd believe her. "I love you, and believe me I want to, its just that I've never…"
Soft fingers touched her cheek, opening her eyes she once again found herself gazing back into his yellow ones which contained a mixture of emotions.
"It's ok I haven't either" He said, pulling her back into a tight embrace, "I'd be lying if I said I wasn't nervous".

"Anyway" he continued, "we can go as slow as we need", leaning in HB pressed his lips against hers in slow torturing kiss that increased the ache she felt for him into a burning need. Returning his kiss with equal passion her hands dropped to his chest and shoulders feeling his well—defined muscles moving beneath his skin. His tail caressing her legs as their tongues tangled together drawing pleasurable moans from each other. Each kiss becoming more heated then the last, quickly making her forget everything around her. She was ready for this, she loved Red more than she could ever say and she needed him so much right now. The rush of emotions coursing through her made controlling the fire within difficult but she pushed it down, Red may be fireproof but his room and cats certainly weren't.
Feeling his flesh hand slip under her shirt, she pulled away for a moment so she could remove it, adding it to the pile of his own beside the bed, revealing her bare breasts to him. "So beautiful" HB mumbled, as he kissed her again, his flesh hand tweaked and massaged her soft peaks in a way that drove her mad. Moving forward so she was straddling his lap Liz could feel his arousal straining against his leather trousers.
Keeping his stone arm tightly around her, Red suddenly stood up, making her squeal excitedly. She quickly wrapped her legs around his waist as he turned to lay her down on the mattress, moving his arm to position himself so that his weight didn't crush her. An animalistic growl rumbled through his chest as she gently bit his neck, smoothing the bite with her tongue. Very quickly the rest of their clothes were removed, their bare legs tangling together. Reaching down Liz stroked him from root to tip, revelling in the sounds that it coaxed out of her demon lover. He felt so large in her hand for a brief moment the nerves were back as she doubted whether or not they would fit. Hellboy felt her pause, "Are you sure you want to do this?" He asked, "I love you and I'll understand if you've changed your mind".
"I love you to Red" Liz replied pulling him down, so she could kiss him passionately. Sliding a hand down her body to the heat between her legs he slipped finger inside of her making her gasp as he stretched her inner walls, preparing her. His movements causing a pressure to build inside of her, as she trembled with anticipation of the real thing. She pulled his hand away, "I want you Hellboy" she gasped, taking hold of him, guiding him to her entrance. Staring her straight in the eyes Red slowly pushed into her, pausing every so often to give her time to adjust to his size, until he was completely buried inside of her stretching her to her limits. The initial pain quickly gave way to a rush of lust and pleasure. She struggled to control the small tendrils of flames flickered around them both as they began to move together, finding their rhythm. Hands and lips constantly caressing and hungrily kissing any piece of skin they could reach as they drove each other to ecstasy. The pressure inside of her continued until she felt she was about to burst. Red pulled her body tighter against his, lifting her off the bed as he increased the speed and depth of his thrusts. She could feel him start pulsing inside her a sign that he was as close as she was.
"Red!" She moaned as he pushed her over the edge, bringing him to his own release as waves of pleasure overcame them. They continued to move together to try and make the feeling last a long as possible.

When it finally faded Red rolled to the side so he could lie on his back, his arm still around her waist keeping her body close to his as they tried to catch their breath. "Wow" she breathed unable to come up with any other words to describe what had just happened. She couldn't even recall the reasons why she had been so afraid in the beginning, they all seemed so silly now. "You said it babe" Red replied his tail curling around her legs, absentmindedly stroking them whilst he ran his fingers through her long hair. Snuggling close into his side she laid her head against his chest tracing the patterns on his skin, enjoying the peaceful moment. The odd couple had experienced many things in their lives, and no doubt will have many more unforgettable experiences in the future, but Liz knew that nothing was ever going to be as good as it was right now, snuggled up to the man she loved after making love for the first time. The feeling was indescribable.

"Liz…?"
"Yes?" she asked tilting her head up to look at him.
"I love you" he said, "I mean that, I love you with everything in my heart", gazing into his eyes she could tell he meant it, they were so full of love and sincerity it made her want to cry.
"I love you too" She replied, pressing one more kiss against his lips, "I always have and I always will", she giggled for moment as she rested her head against his chest again, closing her eyes as she listened to the strong beating of his heart, the sound lulling her off to sleep "Guess you're stuck with me Red".
"Wouldn't have it any other way" he replied as they both drifted off into a peaceful sleep.