saiki would compare reigen to his father haha theyre pretty similar ("the way i see you... You're kind of like my dad" "R-really?!" "You both have brownish hair and are both whiney. You both like to take advantage of my power but at least my dad is upfront about it... And unlike you, he's been in a happy relationship since he was 19" "...")
Since saiki is usually silent and apathetic, mob would think he's actively hiding his emotions, but wouldn't really know how to bring it up. Saiki would read his mind and more than anything would be slightly worried about all the flashbacks mob is having about his past "incident" and would also feel too awkward to talk about it.
Serizawa probably has the impression that saiki doesn't like him much because he's so blunt but truthfully, aside from mob, he is saiki's favourite from the agency. He has this sort of human respect for serizawa.
At the start, dimple feels kind of smug that saiki is this powerful esper but cant see him. But eventually he gets bored and reveals himself to saiki. He is very confused when saiki seemingly still cant see him. Apparently though its just saiki completely ignoring his existence.
Saiki is annoyed by tome, and feels like shes distracting mob when saiki is already hanging out with him. He gets irritated by it constantly. Tome just wants to know if he can shoot a communication beam towards space and talk to aliens. she would get along better with akechi but she wouldn't understand if he believes in aliens or not because he both talks about all the reasons it makes sense that aliens exist and about all the reasons why they probably don't.
saiki wouldn't mind hanging out with ritsu, but would probably inwardly think he's a bit too dramatic. Ritsu would feel exasperated that reigen has an even younger apprentice that he can use.
When, at the start, reigen confesses that he isn't psychic (word hasn't spread much by then) akechi is highly angered and offended by this, which you can tell by his word vomit. He's very polite about it though.
When out together, saiki likes to stay very close to mob, and maybe even grab onto his uniform jacket unconsciously <3 although in his mind he's still monologuing about how annoying and stupid the world is. Mob doesn't really react to it all. He probably hasn't noticed.
When mob sees that saiki likes coffee jelly, he is overjoyed that saiki is showing his enthusiasm for it. Some day, they will mix coffee jelly and milk together, and while they eat, saiki would decide that even if he doesn't really need mob's help, he doesn't want to ever stop visiting him at the agency <3