You and Dazai have been 'living' with Mori for a while now, so you're almot like siblings. You're both the same age, and when Mori gave Dazai the mission to investigate the magic world, did he send you along with Dazai to a magic school in Scotland called Hogwards.

You and Dazai are shopping in the Diagon alley together with the potion teacher, Mr. Snape.

You watch Dazai trying wands with a borde expression. It took forever and even when he would have the right one would his ability nulifi the magic probably anyway. Mister Olivander appired excited and Professor Snape a bit annoyed

(Next part is from the story)

“Well, maybe… It is strange though. If I didn’t know any better, I’d say you didn’t have any magic at all.” Olivander says in thoughts

Dazai’s eyes narrowed. “About that wand, then—”

“Right, of course! I’ll have to make one custom just for you. Very exciting, really. Not very often a case like yourself comes around.” He paused for a moment. “Three weeks… maybe four. And I’ll need something of yours, of course. Hair always does the trick—”

“How about blood?”

Both Ollivander and Snape startled, blinking at the bandaged boy with twin expressions of something like shock while you don't react at all, it's not like that's unusual for Dazai. The wandmaker dropped his hands mid-way through an exaggerated, demonstrative motion. “For the core… yes, I suppose that would do.”

Severus’ lip twitched.

Good. Dazai nodded, reassured. Hair… he wasn’t sure about hair. But blood could easily act as an extension of his own body, especially if Dazai could figure out how to channel his “magic” into it. He was optimistic that if the wand contained blood from his still-beating heart, something might work. Even if that something was just him having a piece of wood filled with blood. While he was still positive this whole magic thing wasn’t going to pan out… well, he’d made a deal with Mori. Might as well try and get a functioning wand.

Anyway, Dazai was kind of excited to see if it would work, too

“Blood... yes, yes. I think it will do just nicely!” The man considered him with an indecipherable look. “I’ll need a vial, course. I don’t suppose—”

“Ah, have you got one, then?” He pulled the pocket knife from the pocket of his long, dark coat— “A jar?” Snape shot him a quick look—something almost challenging or annoyed.

—and promptly dug the blade through the bandages on his left arm and into his skin.

Again, both parties stilled, expressions aghast. Severus wore a look Dazai was hesitant to classify—something jarred, yet cautious. Seemingly too shocked by his abrupt self-harming, the professor and the wandmaker stood in silent surprise until the white bandages started to dye red.

A drop of blood fell to the floor.