Okay, before anything let me say thank you to the beautiful arwenthemuse for the huge help as my beta. You guys can find her on Tumblr :)
1. She is indeed an engineer.
Most of the galaxy is only capable of seeing Commander Shepard as she is presented publicly: the saviour, the dashing spectre who will show up in her shining armour with big guns and save the day. A few individuals with access to a little bit more information know her by her official position in the Alliance: an N7 infiltrator, specialist in combat and tech.
Liara doubted that even the Normandy crew knew about it, for the subject was never mentioned, and felt like she had crossed some kind of boundary when she found that out.
"I was only intrigued and didn't know if my questions would be welcome. I was also sure I would embarrass myself somehow. I apologise if I have offended you by doing this little research, Commander." she had said, avoiding looking at Shepard's eyes for reasons she didn't know.
"It's fine, Doctor T'Soni. It's not like that's a big secret or something. But don't be afraid to ask me anything; I promise I don't bite." Sweet, innocent Liara had not understood the wink nor noticed the lascivious tone Shepard used, but blushed nevertheless.
After their conversation, it was all too easy to realise Shepard's interest in tech went beyond the battlefield. Kind as she was with the entire crew, she made sure of going through every deck, checking up on everyone (especially those who went on ground missions too), but always seemed to leave engineering for last, and stayed there for the longest. Her friendship with Tali developed smoothly and fast, and more often than not they would be seen in the mess hall over meals discussing theories and planning new things to improve either the Normandy or the equipment they used on ground.
Later on Liara discovered a secret pleasure: watching Shepard get enraptured by a new engineering concept and sparing some of her scarce time to work on her skills. She often remarked that for someone who loved to call her a nerd, Shepard was not far from being one herself.
2. She loves being marked.
This one Liara found out during a meld, but was unsure of how to proceed. She knew Shepard was possessive, as the Commander herself had admitted.
"Sometimes I feel almost guilty for taking so much pleasure in knowing that I was your first and only mate. It sounds… primitive, but I enjoy it too much to care."
However, it filled her with a fuzzy feeling to hear Shepard say, "You know you're allowed to mark what's yours, right?" immediately after Liara apologised for leaving a rather clear mark on the Commander's neck.
It did not help that Liara herself was rather possessive, and Shepard never made much work of covering any mark, instead wearing them like one of her medals. The dress uniform she wore did the job for the most part.
Once, Tali had caught her in the mess hall at night cycle wearing more casual attire: sweatpants and an N7 t-shirt that did not cover an inch of Liara's masterpiece. Shepard showed no shame, instead grinning proudly even as her friend made jokes.
3. Sometimes, her scars show.
Right after their meeting on Mars, once they were back on the Normandy, Liara entered Shepard's cabin only to be surprise by a datapad flying in her direction. She barely stopped the object mid-air with her biotics before setting it down carefully on the desk.
"Liara! I'm so sorry, I… I'm sorry."
Shepard stormed around the room for a second then sat on the ground, leaning against the bed. Liara approached her slowly. "It's fine, you didn't know I was coming."
She was looking down the entire time, but looked up when Liara sat by her side. That was the first time Liara saw a visible change on Shepard's body after the Lazarus Project and she did not know exactly what to think of it. Pools of red marked Shepard's green eyes, three on each, like the mathematic symbol "therefore".
"Shepard… what's happening?"
The commander managed to explain everything. How, by the time she awoke her scars weren't completely healed yet, and how her psych profile affected its process, even showed an old email from Dr. Chakwas.
[…] the old adage of "mind over matter" holds true. Negative attitudes and aggressive acts create adverse reactions with your cybernetic implants, while peaceful thoughts and compassionate actions promote healing.
"I never accepted the offer because I knew I didn't need it. It is also a bit of incentive; I'd like to maintain my pretty face as it is. By the time we met on Illium the scars were gone, but if I happen to get too angry I still get this." she pointed with an exasperate sigh. "I never had much problem, really, except when dealing with the whole Shadow Broker thing and now. I'm so angry at the Alliance. And the rest…"
"How did I not notice it then?"
"You were too worried with rescuing Feron," was the answer. Liara felt a pang of guilt even though Shepard's tone was not accusatory. "I guess not punching him in the face like I wanted helped a bit." she joked.
4. She is possessive.
The Normandy was heading to the Citadel after a few recovery missions and they would stay docked for at least one day. The crew could finally take a moment to breathe, so when Shepard called Liara up to her cabin, the Doctor thought she wanted to discuss the project she had mentioned before.
She most definitely did not expect to be pushed against the aquarium and have her lab coat literally ripped apart by Shepard's hands (Cerberus be praised). The exclamation about to leave her lips was silenced by the forceful kiss and instead all she did was moan.
They didn't even make it to the bed, and Shepard was only satisfied once Liara was thoroughly ravished and exhausted, leaning on her to keep standing. Hoisting Liara up by her hips, Shepard carried her to the bed with utmost care, stripping of her own clothes before lying down.
"What brought this on? Because if it was something I did, I'll make sure to do it again." Liara asked with a sly smile.
"You know what I just heard down in engineering? Donnelly said he was thinking about inviting you to play strip poker. Strip poker!"
Liara hid her face against Shepard's neck, trying not to laugh. "Hmm, are you jealous, Commander?"
"It's more than enough having James drooling over you the entire time, I don't need to hear anyone on my crew saying how they want to see my bondmate naked."
"Ah, so you rip my clothes apart." Liara mused. "May I know how I'm supposed to go back to my office?"
"To begin with, you're so not leaving this cabin anytime soon. And when you do, you will wear something of mine."
5. She was the universe's magnum opus.
Liara could never explain that one thing: how Shepard was what she was. Not human, but a vessel of raw, pure power. Uncorrupted, the purest soul to exist. She felt it during melds, and she saw it in Shepard's every action. She was ultimately selfless, and the impossible did not apply to her. She had survived where any other would've perished.
The galaxy may know part of it; they can see the hero, the legend, their saviour, but Liara can see and touch the woman under the armour. Liara can touch the soul guarded deep inside Shepard's body and understand her like no other.
She was granted permission to become one with Shepard, and the things she knew…
Shepard was a woman with super powers and simple needs, and Liara, for all her noble education, could not find the words to explain how such phenomenon was possible. Shepard saw good where Liara thought there was none left, and when all Liara felt was corruption running along her veins.
Liara had been broken, emotionally detached, and ruthlessly carving her way towards her objective and Shepard came back to fix her. She pushed and shoved and stripped Liara of her every protection. She came back from the dead and she fixed all wrongs.
For Liara, that was a true Goddess.