First, I must pause to howl with laughter at your icon. That's a good one.
Ok, onwards. The rules don't really do much good when it comes to actual interactions, do they? It has to do with the personalities, as well as relative social positions. The use of family names (no title) among members of the same seniority level (class, grade of employment, etc.) seems more or less "we are decent friends/teammates/not girls". But Eiji gets called by his given name because he's so darn bouncy and outgoing, and both assumes and accepts a higher degree of informality/intimacy. You might have noted, he calls people by nick-names a lot (Fujiko, Taka-san, Ochibi). From Momo, though, it's Eiji-senpai, which is less formal then, say, Fuji-senpai, but keeps the vital acknowledgment of a difference in seniority. One of the reasons people like to assume Shishido and Ohtori, of Hyoutei, are lovers, is that, while Ohtori is quite proper about calling all his other seniors FamilyName-senpai, he calls Shishido 'Shishido-san', and Shishido calls him Choutarou. Given name alone is more intimate than standard Senior-to-Junior, which would be FamilyName (no title), or GivenName-kun, if they're a bit closer/politer. Similarly, Yagyuu, of Rikkai, is considered to be holding up his nickname of The Gentleman when he calls his same-year partner 'Niou-kun'. So, anyway, my guesses:
1. I would expect Momo's usage to change more than Ryouma's. Ryouma uses the forms without, clearly, investing much actual respect in them, so I wouldn't expect him to feel called upon to change the form in order to reflect, say, a closer friendship. If they were romantically involved, I might expect him to call Momo 'Momo' in private. Moreso, as Momo seems to like his nickname better than 'Takeshi'. I would expect Momo to move from 'Echizen' to 'Ryouma' as they became closer, either as friends or as lovers. He's an affectionate sort of guy, albeit surprisingly proper with his own seniors.
2. Oh, the most fun. I would expect Kaidou to call Inui 'Senpai' or 'Inui-senpai' unless and until Inui suggested that it would be more appropriate to call him by his given name. Since I don't think that would be appropriate until they were lovers, that's when I would expect Kaidou to alter his form of address, probably blushing hugely all the while. Inui is harder. I think he would, knowing how it would disconcert Kaidou, call him Kaoru either as they became close friends (erm, supposing that to be possible) in order to tease him, or else as they were becoming lovers, in order to emphasize the intimacy of the moment.
3. Hmm. Kawamura is so sweet and retiring, as long as he doesn't have a racquet in hand, that I wouldn't expect him to use Fuji's given name unless they were lovers. I expect he would call Fuji-the-close-friend 'Fuji', still. And 'Taka-san' is so much a nick-name, or even pet name already, I would expect Fuji to keep using that for Kawamura-the-close-friend. Definitely 'Takashi' if they were lovers, though. If only to make him blush.
Looking back through that, I realize that I assumed physical intimacy; let me add that I think it would work the same for non-physical romantic involvement.
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Ok, onwards. The rules don't really do much good when it comes to actual interactions, do they? It has to do with the personalities, as well as relative social positions. The use of family names (no title) among members of the same seniority level (class, grade of employment, etc.) seems more or less "we are decent friends/teammates/not girls". But Eiji gets called by his given name because he's so darn bouncy and outgoing, and both assumes and accepts a higher degree of informality/intimacy. You might have noted, he calls people by nick-names a lot (Fujiko, Taka-san, Ochibi). From Momo, though, it's Eiji-senpai, which is less formal then, say, Fuji-senpai, but keeps the vital acknowledgment of a difference in seniority. One of the reasons people like to assume Shishido and Ohtori, of Hyoutei, are lovers, is that, while Ohtori is quite proper about calling all his other seniors FamilyName-senpai, he calls Shishido 'Shishido-san', and Shishido calls him Choutarou. Given name alone is more intimate than standard Senior-to-Junior, which would be FamilyName (no title), or GivenName-kun, if they're a bit closer/politer. Similarly, Yagyuu, of Rikkai, is considered to be holding up his nickname of The Gentleman when he calls his same-year partner 'Niou-kun'. So, anyway, my guesses:
1. I would expect Momo's usage to change more than Ryouma's. Ryouma uses the forms without, clearly, investing much actual respect in them, so I wouldn't expect him to feel called upon to change the form in order to reflect, say, a closer friendship. If they were romantically involved, I might expect him to call Momo 'Momo' in private. Moreso, as Momo seems to like his nickname better than 'Takeshi'. I would expect Momo to move from 'Echizen' to 'Ryouma' as they became closer, either as friends or as lovers. He's an affectionate sort of guy, albeit surprisingly proper with his own seniors.
2. Oh, the most fun. I would expect Kaidou to call Inui 'Senpai' or 'Inui-senpai' unless and until Inui suggested that it would be more appropriate to call him by his given name. Since I don't think that would be appropriate until they were lovers, that's when I would expect Kaidou to alter his form of address, probably blushing hugely all the while. Inui is harder. I think he would, knowing how it would disconcert Kaidou, call him Kaoru either as they became close friends (erm, supposing that to be possible) in order to tease him, or else as they were becoming lovers, in order to emphasize the intimacy of the moment.
3. Hmm. Kawamura is so sweet and retiring, as long as he doesn't have a racquet in hand, that I wouldn't expect him to use Fuji's given name unless they were lovers. I expect he would call Fuji-the-close-friend 'Fuji', still. And 'Taka-san' is so much a nick-name, or even pet name already, I would expect Fuji to keep using that for Kawamura-the-close-friend. Definitely 'Takashi' if they were lovers, though. If only to make him blush.
Looking back through that, I realize that I assumed physical intimacy; let me add that I think it would work the same for non-physical romantic involvement.