Sigroon joined your party about halfway through the game after returning from the Ruins isle, and stayed through three dungeons/boss fights, after which Aisha rejoins your party. Just like Mustafa/Cruxie, her stats and skills were a copy of Aisha, and Aisha picked up where Sigroon leaves off when she rejoins.
I used Adol/Dogi/Aisha. Aisha was so broken. I couldn't stand Sigroon for some reason, and during that period, I used Geis. I used Mustafa early on, but the change to Cruxie annoyed me so I stopped using that moveset.
My party are Dogi,Adol and Aisha. The character I use to run and fight is Dogi(I change him to Adol or Aisha when I have no opportunity with the monster). I didn´t use the other characters so much,and I will NOT use Geis.I hate that guy.