Kefla is a very good unit, she's a nuker that provides orbs for herself and she has a built-in dodge and additional attack in her Passive.
How to Get
Obtain through Rare Summon. [ JP version only]
Causes immense damage to enemy and lowers DEF
12 Ki Multiplier
"Potara" Category Ki +2 and HP, ATK & DEF +120%
Change Ki Spheres to Ki Spheres; ATK & DEF +10% per Ki Sphere obtained; medium chance to evade enemy attacks; guaranteed to get an additional attack and medium chance for that additional attack to be a Super Attack
Kefla can be used on two teams so far: "Potara" and Super PHY.
"Potara" Kefla should mainly be seen as a filler unit on this team, unless of course you are using her as the leader: she should be left off-rotation since she doesn't synergize that well in terms of links with the rest of the team and she takes away INT orbs from [Super Powered Fusion] SS Goku & SS Vegeta. Fortunately enough, the leader of the team gets more Ki from rainbow Ki spheres, so her orb-changing Passive is not the worst, she can either use those orbs to nuke or leave them for another unit.
Super PHY Kefla's role on Super PHY is not to be the orb-changer, if you were to choose a unit for that Unconventional Fusion Whirus would be a better choice: her lack of common Ki links can turn out to be helpful not throw Super Saiyan 3 Gotenks [Naught but Rampage] overboard with the Ki. She should still be left off-rotation, the element is stacked with great units that take that spot, but that may change when she finally gets a Dokkan Awakening.
Kefla has an immense damage multiplier and greatly lowers the enemy's DEF, which is not that relevant of a thing outside specific Dokkan Events like the LSS Broly's one.
Kefla's Passive changes INT orbs to PHY, which is great since she is a nuker, getting a 10% boost to ATK and DEF per PHY orb collected. The second part of her Passive gives her a 50% chance of evading the enemies' attacks and an additional attack, all without having to touch the potential system.
Links are the downfall, "Super Saiyan" and "Fued Fighters" are the only good links, the rest doesn't really find a place anywhere. "Tournament of Power", "Warriors of Universe 6" and "Battlefield Diva" may be useful once her and [Perilous Potential Strength] Kale (Berserk) get a Dokkan Awakening and maybe a good category leader for "Universe Survival Arc".
In terms of Ki these two units link very well together, "Warriors of Universe 6", "Battlefield Diva" and "Tournament of Power" add up to 7 Ki, giving them an automatic Super Attack, but Kale (Berserk) does not have "Super Saiyan", the only relevant ATK links SS Kefla has.
While not sharing anything aside from "Fused Fighters", having both of these two units on rotation can change three different colours of orbs to PHY, allowing Kefla to maximize her ATK and make the most out of her Passive without having to use any item.
The best build for this unit would be lv.11 Additional and lv.15 Critical since she already gets Dodges and a natural lv.5 Additional in addition to the free one from her Passive.
Should I use Elder Kais/Grand Kais?
SS Kefla may become farmabe in future, you can invest Kais on her, but we don't know when she's going to get a Dokkan Awakening, holding off would be a good choice if you don't have that many to spare.