Bellinda's case:
(On having the said unit on your team roaster you get to enjoy there passive ability..)
For example: You use a team with Bellinda+Cloris...
on the map you deploy Cloris first...
now she gets the attack buff of Bellinda's passive...
next you deploy Belinda...
say Bellinda dies after sometime...but Cloris survives...then also Cloris will continue to get the buff..
You don't need to have the unit on the field to enjoy the passives of certain type like
'Drop rate boost' passives
'Class stats buff' passives etc.
While in Lesia's case...
You need to have the unit deployed and alive on the battlefeild to get her passive ability...if she dies or isn't deployed her ability isn't in use...
Such type of Abilities are mentioned with a quote: 'when deployed on the battle field'
Other examples of such type of ability is
Altair(black soilder) and Berna(black rouge)
They can automatically 'withdraw' units of there respective classes...
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Don't waste your (nutaku: DC) magic crystals on spirits...at the start you will feel a rush to cc your units but remember that Aigis is a slow paced game...it will certainly take some time but after you have played for few months you will be getting platinum fairies from constant source: Events/Spirit Rescue or Phalnax 2...
Its not worth 70 DC just for a insignificant plat fairy :P
Actually you should save all your DC and spend it wisely especially because there are lot of TP units in DMM version because of monthly TP rotation.