Yoshimo is Tamoko's brother and originally came to Faerun to search for her, after she ran off with Sarevok.
His whole sub-plot doesn't begin until AFTER he dies, since you were supposed to have the option to tell the priest of Illmater of the circumstances holding his soul in limbo after death, which starts a questline to break the Geas he's under, at which point you got the option of letting him go on to the after life, or reviving him so he can have a chance at revenge against Irenicus. At which point, he opens up about his past and why he came to Faerun in the first place, since he's no longer bound by the Geas.
And once he and Sarevok meet in ToB...well...it's not exactly a happy reunion (He either blames Sarevok for Tamoko disappearing, or getting her killed, if you admit earlier that you killed her (If you did) while she was defending him. He doesn't blame you for it if you did, since it was only to be expected that she would defend the man she loved, even if he was just using her).
Which goes off in 2 different directions depending on if you do or don't redeem Sarevok. unredeemed is fairly unrepentant about the whole affair and eventually leads to Yoshimo attacking him, and while redeeming him causes him to open up and try to make amends with Yoshimo, who does eventually forgive him.
(If you cheat and add Yoshimo to the party with Sarevok, at some point after he's redeemed, he has two conversions where he's addressing Yoshimo about Tamoko and his remorse over the whole affair)
Yoshimo also has several conversions that occur after he can no longer be in the party without cheating/exploiting (Such as in Hell or right before the Assault on the top of the tree).
Some elements of their joint ending were merged into Sarevok's ending, mostly the part about him returning Tamoko's ashes to Kara-tur, if she died (otherwise he and Yoshimo walk the realms trying to find her), and the redeemed Sarevok realizing what a MASSIVE mistake he'd made by letting his ambition rob him of what he had at one time genuinely cherished.
Several of the developers will openly agree that the above is that was intended, but they ran out of time and it ended up getting cut (I blame Imeon, personally, since they had to suddenly add a bunch of stuff for her last minute because of people getting annoyed at her being killed off in Spellhold, even though she was originally supposed to be just a cameo from the previous game to help kick off SoA's plot).
Comments
and you are going to assylum to find Imoen with still dead yoshimo
It would be interesting to see her and Yoshimo have banters...
I wish, I've been wanting Yoshimo's full questline for years, after I found out what it was supposed to be like.