Mimitchi is a female on the V4 - but can be a male or female on V1, V2 or V3 tamas.
Tamabloom12 and tamagirl34312 have described how to get a Mimitchi on the V4, so I will just add that to get a Mimitchi on the V1, V2 or V3 you need to take very good care of your teen character and try to keep the Happy and Hungry hearts as full as possible at all times during it's growth.
This usually ensures a Mimitchi. The only other thing to be aware of is that the Mimitchi appears on odd or even generation tamas - depending on if you have a V1, V2 or V3...