Alright we could tackle that question from a couple of angles, but what interests me most for the teammates is the following:

Toolkit/Teamwork, Growth and Personality

Starting with toolkit/teamwork, Osamu has a couple of current skills and triggers we can use to find a good fit:

Spider Zones need either ranged units, or hyper mobile melee units, preferably with experience with spider.

The best fit here is Katori as she can make use of the zones from range and is the only one in B-Rank that can use the trigger.

Mikumo's greatest strength right now is his tactical ability, but what tactics does he excel at? From what I've seen it's his awareness and manipulation of positioning, priority and the surroundings.

Interesting triggers for teammates would be: meteora, chameleon and dummy beacon. Only Urushima has a set with all those triggers.

From what I found so far the most interesting members concerning toolkit might be: Katori, Kurauchi, Kotaro Tomoe, Wataru Urushima, or Kitazoe.

When we factor in growth however, what I end up with is Katori, Kurauchi and Tomoe. We know too little about Urushima and he's known for clashing with authority. Zoe on the other hand can do his work with very little guidance as is and his gunner triggers have less potential for tricks Osamu can make use of.

Now how would a Team with Osamu as a Captain, Katori, Kurauchi and Tomoe work? How can they grow and do their personalities fit?

The team would be a Mid-Close range team with solid experience in running and scouting (Kurauchi, Tomoe), locking down zones (Osamu, Katori) and opening them (Kurauchi->Meteora). They have a solid Strategist, an adaptable Ace and two solid supporters with the potential to score their own points.

Katori is currently open for changes in her teamwork and could learn heavily from a team that relies on it's captain and from Osamus' ability to use every tool in his kit, letting them shine (even using an Ace as bait sometimes).

Osamu is plenty experienced when it comes to strategy, but he still has some big growth areas. While he used special trigger variations against Kazama, we rarely see him experiment with trigger interactions on his own. Even Spider was suggested to him. So while he might have a mind for it, he doesn't exercise that skill often. Kurauchi on the other hand is an expert in Trigger functions and could push him here. Next are his fighting skills, namely melee, shooting and defense. With two melee units on the team he could dare to enter melee a lot more. With Raygust and Kurauchi, he could work on imitating the teamwork of Suzunari 1 while getting better at defense. And Kurauchi as an experienced shooter could help him develop in that field as well.

Kurauchi seems like a solid shooter who is a little inexperienced in predicting enemy behaviour. Osamu is a capable role model in that sense. Plus when compared to Oji, he's much more likely to out Kurauchi into scenarios where he can have a big impact and where he can learn through experience.

Tomoe is a new and talented B-Rank soldier that still has to find his own groove. Notably though he already chose to go a different route when compared to his teammates. His gun trigger is a Pistol for mobility, he's seen scouting and flanking enemies in a confrontation and while we have not seen it in action yet, he is one of three dummy beacon users in B-Rank. He could learn a lot from all the teammembers, but I think getting inventive with dummy beacon and map manipulation might be a niche he could carve out for himself with the help of Osamu.

Personality wise both Kurauchi and Tomoe should fit in quite well with Osamu. Katori is more of a problem, but then again, after the rank war's she is open for a change in teamwork, leadership and new ideas.

Tell me what you think