School Rumble doesn't have a stable plot in the first four volumes. Most of the chapters are one-shot gags. It isn't until Eri and Mikoto have problems with each other in the fourth volume that it shows some semblance of stability, and the characters that get center stage for that are Eri and Mikoto. The next major arc is the sports festival, and the protagonists for that arc are Eri and Harima. Next is the Battle Royale parody, which has Hanai as the main protagonist and Tenma as a secondary protagonist.
Following that is the play arc, where Eri plays protagonist to Yakumo's antagonist. A basketball arc occurs after that, where Asou and Mikoto are the main characters. Then, we have a manga arc focusing on Yakumo before the Omiai arc, in which Eri and Harima are the protagonists. Kyoto arc is next, and Eri and Harima are the protagonists of that, too. Then, we have an arc for the feud between Eri and Tenma, where Eri is the protagonist antagonized unknowingly by Tenma.
The storyline of Tenma's quest for Karasuma has been underlined throughout all this, as well as Harima's pursuit of Tenma, and the two serve the purpose of bridging all the mini-stories together. Yet, almost never in the story does Tenma actually act as the protagonist, and Harima's relationship with Tenma has almost always been overshadowed by his relationship with Eri.
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