Steiner learns abilities that enhance his abilities as a physical fighter, such as Accuracy+, Long Reach, MP Attack, and Killer abilities. His also learns Counter and Cover. He doesn't learn any unique abilities.
Steiner's Sword Arts include stat breaks and several powerful attacks, such as Climhazzard and Shock. Due to the Thunder Slash glitch, Steiner's Sword Art skill Thunder Slash has an accuracy of 0%, rendering it rather useless, although with certain equipment the percentage can be raised, and the attack can be used against enemies weak against thunder, such as the Iron Man.
While Vivi is in the party, Steiner can use a version of Sword Magic to enhance his attacks with magical powers, but the kind of magic applied to his sword is directly related to what Black Magic Vivi has learned, i.e. Fire Sword is only available if Vivi knows the Fire spell. Magic Sword only uses Steiner's MP. His Sword Magic is the only way to use Doomsday without it hitting the party as well.
During the performance of I Want to Be Your Canary at the beginning of the game the player has the option to find the Knights of Pluto as Steiner. Locating them all and reporting to Breireicht in the left tower will earn an Elixir.