My brother and I have been putting a lot of time into Megaman Starforce 2 (both versions) for the Nintendo DS over the last week. I have been a big fan of the Starforce series and the original Megaman Battle Network series for years now. However the two main characters of Megaman Starforce, Geo Stellar and Omega-Xis, have become my favorite characters out of the entire combined series. As a pair of characters, they regularly produce fun scenes and hilarious dialogue. Without their interactions, the two Megaman Starforce games would not be anywhere near as fun.
In the original Megaman Battle Network series, the equivalent characters to Geo and Omega-Xis were Lan Hikari and Megaman.exe (otherwise known as Hub, Lan's twin brother). Like most Net-Navis in the Battle Network series, Megaman.exe was in many ways an exaggerated or slightly distorted mirror-image of his operator. In general, Lan was the slightly more aggressive and confident of the two characters, while Megaman.exe tried to act as Lan's conscience. However, the two were both pretty straight-forward heroic characters, and thus were generally in sync with each other. Unfortunately, this meant that the character interactions between the two were often somewhat bland. While there were some truly touching scenes built around the fact that they were two brothers (found mainly during endgame sequences), many of their regular interactions often boiled down to Megaman.exe nagging Lan to do his homework.
In comparison, the conversations Geo Stellar and Omega-Xis have even during routine gameplay range from mildly entertaining to downright hilarious, thanks to the strongly contrasting personalities of the two characters. On one hand, Geo Stellar fits perfectly into the reluctant hero archetype: he is a rather timid boy who lacks drive and confidence who is dragged into dangerous situations against his will. However, Geo possesses strong morals and dreams about becoming someone like his father. In comparison, the alien warrior Omega-Xis is an incredibly aggressive character. Omega-Xis thrives on combat and excitement, is always eager for a fight, and has no qualms about doing things like poking around people's secret data files. It is difficult to imagine two characters more different than these two.
These contrasting personalities fuel a huge range of funny and entertaining interactions and conversations. One of the most distinct is the hilarious scenes where Omega-Xis (who dwells in the computer Geo wears on his left arm) literally dragging Geo around by the arm because Omega-Xis got bored and wanted to go somewhere more exciting. We see Geo protesting when Omega-Xis wants to invade someone's computer to take a peek at people's diaries, before Omega-Xis ignores him and does so anyways. And we see Omega-Xis constantly teasing Geo about the various girls who have crushes on him. Because both characters have such contradicting perspectives on everything, it is easy to write a wide variety of interesting material for them.
Perhaps the most useful outcome of Geo and Omega-Xis's personalities is that it gives a very good justification for why a reluctant hero like Geo would actually become a game's main character. In Megaman Starforce 1, Omega-Xis makes it clear to Geo from the get-go that he knows something about what happened to Geo's father, who mysteriously disappeared some time prior. So, Geo has a very good reason to put up with Omega-Xis. From that point, Omega-Xis simply drags Geo from one dangerous situation into another until Geo has slowly become accustomed to being a hero. So there was no point in the game where I asked myself "So why is this guy the main hero?".
In the end, I think the direction the developers took with the personalities of Geo and Omega-Xis was the right one. It has produced really entertaining results so far. While the risk that the writing for the game will get stuck in a rut still exists, I hope that these two characters will be able to avoid that fate.