Mika Koizumi (小泉 ミカ) was the original Yellow4 of the Biomen. She was a photographer who dreamt of following in her brother's footsteps to photograph the wildlife in Africa.
Initially, she is hesitant to join the Bioman team. However, she is convinced when she realizes that if she does not fight, then even the animals in Africa that she loves will eventually be in danger. For someone who is hesitant to do battle, Mika is a powerful and skilled martial artist. In Episode 10, she becomes a martyr to save the others, and dies from several repetitive blasts of Anti-Bio Particles from the Bio Killer Gun. The team later gave her a proper funeral. In the last episode, Peebo apparently forgot to recall her for some reason in the script though Peebo remembers her during the first appearance of Silver.
Mika is a bit of a loner at times and can be a bit stubborn. Although she has a good heart it is shown despite her attitude at times Mika is loyal to the rest of the team even if at times she doesn't show it. She also has the habit of taking pictures at the weirdest times (such as when Biorobo grabbed her in the first episode) and also appears to be a bit of a UFO enthusiast.
- Biojet 2 (shared with Green and Blue)
- Techno Brace
- Bio Sword
- Thunder Sword
- Bio Machine 2
- Super Electronic Holography
- Yellow Flash