ooo-kay. here's where I explain the existence of leilani.

mahana olda was cruising around on her chopper (I'll 
take some pics later) when she happened upon a small 
cavern filled with liquid protodermis. little did she
know that makuta had used the same pool for creating 
rahkshi out of kraata. well, anyway, mah fell in and was
turned into mahana-sorta-nuva. she was exactly the same,
except that she gained nuva armor. 
well, it turns out that the last kraata in the pool left
a bit of its "evil" residue behind, tainting the protodermis
and splitting some of mahana's darker side away from her,
creating leilani.
on to the fun stuff. given her part-rahkshi background,
lei has super-poseable rahkshi legs. she has also been
given the silver nuva weaponry that mahana should get. 
this takes the form of her voidrazors, twin shuriken-like
blades that can not only be thrown and return like a
boomerang, but also create huge vortexes that trap enemies.
when fused together into her buster blade, she can rip
entire mountains apart.
leilani sports the kanohi tere, great mask of agility.
with this power, she can perform amazing feats of 
acrobatic skill, such as dodging enemy fire and unleashing
a bruising physical onslaught of punches and kicks.
she was originally thought to be the toa of void because
of her vortex-creating blades, but upon mastering her 
capabilities, she is revealed to be the toa of chaos, which
makes for a very reasonable match to energy, if you study
chemestry at all. she can take any creature, vehicle, toa,
turaga, matoran, or what have you on the island and 
mix their parts up to the point of complete irrecognition.

in my happy little world, this takes place at about the 
time of the mask of light. L seeks out makuta on her bike
in order to find out why she exists. in order to slow down
the toa, she scrambles ussanui into a hulking colossus
beast with jaller's mask while she speaks to makuta.
he says she has nothing to do with her and that she's 
basically meaningless, so she returns to the shdows to sulk,
returns ussanui to its rightful form and lets taka take
over from there. 
later on, Mahana uses Hahli's Unity charm to bring herself
and leilani back together, creating the true mahana nuva.