Evolving a girl will unlock her H scene.
There are some as noted in the first post of this thread...
Lucy Far <- trash tier
Okabiki Meguru <- noidea, I don't have her
Ultimiya <- trash tier
Thomas Edison <- pvp only
Kurogane <- noidea, never used her..
Space Pirate Sapphires <- low dmg but the fastest attack speed in the game, good for pressure
Shadmalice <- slow re-attack but high reach make her good agains anything that you don't want to face-tank (ridiculous in swarms)
Invadia <- trash tier
Gakutensoku <- trash tier
Ne-ko <- without her other version shes just trash tier, noidea if it would really help to have her other version around..
Oda Nobunaga <- Minamoto does her job better
Asmodeus <- dangerous if used as a swarm, or behind a REALLY big meat shield
Y-Poola <- best R rush unit (has warp after evolving)
Ona no Takamura <- trash tier
Yarma <- trash tier
Tamaki Tamaki <- below trash tier

Hybrider-1 <- fastest R unit in game, not too useful for actual combat (combining her with the summon restrict power against bosses is useful)
sidenote: the usefulness of R rarity ranged damage dealers is totally and utterly out-shined/outperformed by Elfella to the point where using them in her place is just not an option.
- A good flier <- not really something you would need, srsly, their only advantage is that they are not affected by the slowing tentacle power(pvp.. but noone uses that anyways)
- A good ranged girl <- Elfella after evolution
- A good heavy hitter <- if its high dmg in single attack that you're looking for then Shadmalice/Asmodeus, if its dps then Shappire has the highest DPS of all melee R units, if its ranged dmg then Elfella
sidenote: don't forget that placing multiple copies of the same unit in your roster will reduce their summon cost.
Also, what is a good stage for level and money grinding?
Any boss stage as long as you stay long enough, preferably one that has R girls as enemies. (12-6 for example)
Explanation : you can stay in one map for up to 20 mins, you lose the ma at 20 mins but staying for 15+ mins helps getting more drops/gold, boss stages dont have the problem of winning it unintentionally too early (just don't beat her minigame when you reach her until you farmed enough..), when you kill an enemy unit you get gold based on the rarity of the unit (NPC-30 NGirl-50 RGirl-250 SRGirl-750), so a map with mostly R/SR girls as enemies gives more cash over the same amount of time..