Lulu? You're obviously not doing Dark Aeons or Monster Arena if you find her useful...
Kimahri's best path is Rikku's Learn Steal & Use then go on Wakka's. Kimahri, with fully charged Celestial Weapon is a Life Saver with the Monster Arena Stat Max because of his Counter capabilities. I used Lulu the Least, infact I never used her once at High levels & against the Uber-Fiends, she's absolutely Useless, no matter how funny it is seeing a small Onion hit for 99,999 damage the novelty wears off & ur left with a mediocre Celestial Weapon & a laughable overdrive. She's very useful early on, but when you've reached Zanarkand, dispose of her.
I stress again how important Kimahri is for stat maxing, Very.