A couple of weeks passed, and Mandy still never left her room. Her parents were too scared to go check, and Grim was no longer able to look through her window since the curtains were now closed. He just heard her talking so now and then.
It's not like this was the first time Mandy did something dubious, but this time it worried Grim.
She tried to take over the world more than once when she was 10, but at one point she just stopped trying, and neither did she talk about it ever again. Untill now.
She might have a very solid plan.

Next to the sudden plotting, Mandy has been keeping herself on a certain distance from Grim and Billy these 5 years, too. More than usual. Even though Billy never seemed to care about any of this, Grim feared the worst.
It's strange for him to be afraid of something he isn't even certain of; what if she's simply no longer interested and is just polishing her nails right now? Most likely not, but still.
He has always had a feeling for these kind of things, but what if he's wrong? It will only make him look silly if he'd barge into her room demanding for answers when she's just reading comics.

He decided to no longer give any attention to the matter, he had better things to do. And he didn't see how this would be bad for him, he's the Grim Reaper, for Pete's sake, and he's already forced to be her and Billy's best friend forever. Can it really get any worse?

Mandy's parents, on the other hand, had more trouble ignoring this. She was still their daughter, and als long they lived under the same roof as her they were afraid and scared of anything she might be up to. Every day was a struggle to please Mandy. If you're doing it wrong, there won't be another day.

"Mandy..? Mandy..!" Mandy's mother yelled from down the staircase, "Eh, don't you want..something to eat? Mandy?"

A simple "no" was heard from above, suggesting her mother to leave at once.
Claire quickly walked away from the staircase and headed to the living room where she spotted Grim watching television.

"..Grim, can you check on her?"

"You're worried?" Grim replied.

"Worried about my own safety."

"Well, I'd like to, but...no wait, I wouldn't like to. Good day."

"Please, Grim, what if she's making some kind of doomsday device? Besides, those presents from Billy in the corner are starting to smell really bad...wouldn't she like to have them?"

"HAHAHAHAHA!" Grim laughed, "No."

"Then just use it as an excuse to get inside! Remember, if she blows up Earth there won't be any TV for you to watch."

That was convincing enough for Grim to at least dump the trash in Mandy's room, but no way he was planning to stay there to interview the girl. When being around Mandy he still feared for his immortal life. It's strange, but that's what Mandy does with you.
Grim slowly walked upstairs with the giant amount of artwork. He never gave any of it to her since he figured out on his own she wouldn't be interested. Or even slightly flattered by it, for that matter. Really, underwear made of roadkill? Billy had a strange taste of presents to give a girl.
Exhausted, he threw the enormous pile on the floor next to her bedroom door, and knocked.
"..Mandy? Mandy, are you still alive?"

That was a stupid thing to ask, of course she was alive. He's the Grim Reaper; he would've noticed if she was dead.
And it would be party time for him when that day comes. A delighted smirk appeared on his face, untill he woke himself from his beautiful daydream and focussed on Mandy again.
"..Mandy, can I come in?"

"No." said a voice from the other side.

"But I have all dese gifts from Billy. He misses you very much, you know."

"I said no."
This time the voice was sounding more threatening.
But Grim continued, "Well, listen, you can't spend your entire Summer Vacation in your room. Nobody saw you come out for weeks. You are eating someting, are you?"

"I can assure you that I have my own supplies. Now go."

"But-"

"Grim, I'm counting to 3.."

Without a second doubt Grim rushed back downstairs and jumped behind the couch in fear. This conversation didn't make him much wiser, but it's like he promised himself: he didn't go upstairs to interview the girl.
Besides, it's not like it matters that Mandy didn't want the gifts, he's been lying to Billy these past weeks, anyway:

"Yes, of course Mandy liked it."

"Mandy says thank you."

"She said it was the best ting ever."

He could lie a few weeks longer. So far he knew, Billy's been buying it, so what's the harm? He made Mandy look greater than she really is, and Billy loved it. If anything, he's too good for these kids!
Without him their crippled "friendship" would've ended long ago. That's how he saw it.