The development of Sasha and Tomo's relationship in 'Seikon no Qwaser' is something I think a lot of people miss the forest for the trees on because of the show's, uh, very prominent other elements. If you can look past that, it's actually a fairly straightforward shonen-style bond that evolves from a hostile, transactional start into a genuine, protective partnership.
Early on, Sasha basically sees Tomo as a necessary source of power, a 'Soma' to be used. There's little warmth; he's blunt, harsh, and driven solely by his mission to avenge his sister. Tomo, terrified and confused, is just a vessel to him. The shift begins with small acts—Sasha saving Tomo from other threats not just for the soma, but because he starts feeling responsible. The real turning point is when Tomo's own will and courage start to impress Sasha. She's not just a passive battery; her determination to understand him and help him, despite her fear, forces Sasha to acknowledge her as a person.
By the later parts, it becomes a codependent bond of mutual survival and purpose. Sasha's icy exterior melts significantly, showing concern and even a rare smile for her. It never becomes overtly romantic in a traditional sense—the show's framing complicates that—but it's deeply possessive and protective in a way that feels like the foundation of a very intense, dedicated partnership. The relationship's core is them moving from 'user and tool' to 'partners facing a dark world together.' It's clunky and overshadowed, but it's there if you stick with it.