Like with Sonic, Tails's 4th girl is someone from Freedom Planet. (If you haven't heard or played it, do it. It's a fun game.) It's really perfect how much Team Sonic pairs up with Team Lilac. If a fan artists ship Sonic x Lilac, then there are 90% chances they ship Tails x Milla.

The other girls have some dominating presence over Tails since they don't approve of his bad habits of all-nighters (or their Marine) or the different level of shyness. Milla, on the other hand, is writing to be like Tails. They know what others want without really thinking about it. It takes Sonic and Lilac to get them out of the lab more often.