This is always a WORK IN PROGRESS
From the Soul Eater Canon
Maka Albarn
togana-keiri Soul's relationship with Maka is very complicated. She is easily the most important person in his life and her safety and well being comes first, above his own. She's the only person within Facility that holds the power to build him up or break him, her opinion the one that he holds most high.
After seeing how happy she is with Jet, Soul has decided to give them both the ok to pursue a relationship, despite the fact he holds feelings for her as well. He's made the choice to actively do what he can to get over his feelings for Maka and has found the strength to do so through his growing friendship with Rosette.
Tsubaki Nakatsukasa
demonswordmode Tsubaki is a good friend that Soul remembers from home. When he first heard that Tsubaki had found a new partner and boyfriend, he wasn't sure how to feel since he had always seen her with his best friend Black Star back home. Now, he understands how important it is to have a partner in a place like Facility and holds no hard feelings towards her. He hasn't really met her new partner Jin-Ho, but he trusts her choice in men.
Patty Thompson
ircowgirl A friend from home. CR report pending.
Fraken Stein
prideofwisdom A teacher from home. CR report pending.
Tezca Tilpoca
smokingmirrored A deathscythe from home. CR report pending.
Justin Law
guillotine-beat Soul has limited experience with Justin. He had heard that he had betrayed Shibusen back home, and knowing that he's here in Facility doesn't help him relax much. Not that anyone gets to relax in Facility. He expects that Maka's gunna want to try and save him since that's a hobby of her's, and more allies certainly won't hurt. He's wary of Justin and wouldn't approach him unless he needed to.
Giriko
legberjack When Soul first came to Facility, he had just come from a heated battle with Giriko, where it looked as though he and Maka may not pull through. Not long after arriving, they ran into Giriko and Soul and Maka were dealt a devastating loss. Soul is intimidated by Giriko and would never approach him on his own, and recommends that others stay away as well. When Maka first proposed doing something to help 'fix' Giriko, it sparked a number of arguments between them, Soul fearful that it would end like all their other encounters with him, and also for his own hatred for the chainsaw weapon. He doesn't trust him and probably will still distrust and dislike him even if Maka were to reach Giriko.
Kishin Asura
ohfuckitsscary The fact that Asura even exists here in Facility worries Soul. It's one reason why he sticks close to Maka. This is in fear for her safety, but also in fear for how Asura might affect his sanity which Soul already has to work to hold on to at times. He's not sure how to go about fighting him and doubts they have the power they'd need to face him. He fears running into Asura without Maka around to help ground him.
From Other Canons
Jet
rebelliousratAvatar: The Last Airbender
Soul's feelings about Jet are...mixed. He thinks Jet is a bit reckless and puts up a front. He still doesn't quite see what Maka sees in him, but he's had no reason to not to like him. Jet's proven that he cares for Maka's safety and since she likes Jet so much, it's in his best interest to like him. He's talked to Jet about Jet's feelings for Maka and has given him his ok to go ahead and pursue her simply because there's no reason not to. Jet is seen as an acquaintance, a friend of a friend. Still, Soul does his best to treat Jet with respect.
Rosette Christopher
lyrical-lamentChrono Crusade
Rosette is a new friend Soul has made, and he really enjoys her company. Her wonder at his abilities and his home often cheers him up and makes him feel cool. When he's with her, he forgets his concerns about Facility and his worries about his changing relationship with Maka. She's the one he's shared the most with that he's met here. Rosette's a very important friend.
Alex Mercer
livingbioweaponPrototype
Alex is a friend who has been there for Soul since he first showed Alex a little bit of trust. He knows that something horrible happened between Alex and Maka at some point before he had arrived in Facility, but since Maka has forgiven Alex and trusts him enough to be around him, Soul gives him his trust as well. Alex has sort of assumed the older brother role for Soul and out of all the guys he's met in Facility so far, he'd turn to Alex first if he had a problem.
After a fight with Alex's shadow and the near sexual assault on Maka, Soul's trust in Alex has been crushed. He knows that it was the darker part of Alex that had done those things, and not necessarily his friend, but it's hard for him to just forgive something like that. It's likely to continue to be a problem between them for a while.
Helion Lasfort
iai-puppy Helion is a friend that Soul's confided in when he felt particularly left out. He thinks she's sweet and would be glad to help her if she asked. Oh. And they made a child together in the bio dome. But that's neither here, nor there.
Allen Walker
walker-sanD.Gray-Man
Soul trusts Allen and his judgement on things, but the two haven't had much time to spend bonding. Allen's obvious close connection to Maka and Allen's niceness has made him want to grow closer to him. As of now, though, he's a friend of a friend acquaintance.
Alessa Gillepsi
hurtandscaredSilent Hill
When Soul first saw Alessa on the network feed, he worried about the little girl's safety and rushed over to make sure she was ok. Facility's not a place for children. He has no idea what she is or what she's capable of. Alessa, a lot like Rosette, help him feel added importance and he really does care about her well being.