The essential reason is probably to fill up the episodes (TV comes out too fast, manga too slow).

Tenpai was chosen because he was so strong, and it just so happened that the 3rd team was short of a captain.

One of his swords is a zanpakuto and the other is a katana.

(Kabuto is a newly developed sword that generates energy by consuming the user's spiritual energy. So the user must have absolute power or else the blade will devour their soul. And each one will have a different ability.)

His main different ability besides is the Kashira sword. His Kabuto can nullify the abilities of Shinigami Zanpakuto (ahem, except for his own clam of course =v=.) His Zanpakuto belongs to the fire system, which is also very powerful lah.

He does all this for revenge. His father was a Shinigami, but then he was forcibly taken as an experiment to develop the Zanpakuto, and his soul was devoured and controlled by the Zanpakuto. The Chief Captain had no choice but to kill him, and this image happened to be seen by Tenpai, who came out to look for his father, and tried to decide to take revenge. Later learned what happened, for repentance suicide.

ps: this is not a very important character, do not need to care very much, with the main line of the Shinigami plot does not have a lot to do.