Gender, clothes, heat, layers, more heat, more heat..

[Nidan Melbourne]

as much as my sensei allows t-shirts to be worn underneath the gi he would have a limit on what type that you could wear.

As it had to be the same color as your gi with nothing on it or don't wear one at all. Otherwise he would still allow compression garments

[ShotokanMaster]

the ladies in my karate class wears T shirt then their gi sometimes they just wear our karate shirt and the guys they sometimes wear T shirt then their gi or no T shirt at all just the gi I only wear T shirt when its cold season i dont wear any shirt on summer