Masaru Aso was originally an assistant to Doctor Mochizuki until the doctor conducted experiments on him. Horrified at being made into a grasshopper-like cyborg, Masaru went into hiding in the mountains for two years until he awoke with an unconscious urge to protect Hiroshi. Though he hated Mochizuki, Masaru eventually forgave him before learning the man was telepathically communicating with him.
His only attacks are:
- ZO Punch (ZOパンチ Zetto Ō Panchi?)
- ZO Kick (ZOキック Zetto Ō Kikku?)
Another of ZO's features within his insectoid head called a Break Toother (ブレイクトゥーサー Bureiku Toūsā?), a breathing apparatus stored around the mouthplate that shoots out vapors, baring resemblance to fangs when extended.
ZO also rides on the Z-Bringer (Zブリンガー Zetto Buringā?), built by Dr. Mochizuki. Though resembling a regular motorcycle, the Z-Bringer changes along with ZO with ability to reach speeds up to 1300 km/h and resist heat up to 1000 degrees. ZO can use the Z-Bringer to execute the Z-Bringer Attack (Zブリンガーアタック Zetto Buringā Attacku?) Rider Break.
- Dex Stewart - Kotaro Minami's American counterpart in Saban's Masked Rider.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga