Sensei

"“Remember your training, Swordman. Everything will be okay.”"Sensei is a fairly minor character in the Duel series, having only two appearances in the entirety of the series. He is considered a master of dueling, and supposedly taught Swordman and Gunman's family.

Lore
In the main Duel series, he is only known to be a master at dueling. Not much is known about him.

In Duel - The Finale, he stops time and possesses the giant bullet shot by Gunman, calling out Swordman's name. Swordman then realizes it's Sensei and calls out for him. Sensei then tells him to "remember his training" and "everything will be okay." Swordman remembers his training and utilizes it by using one of the abilities given to him by Sensei, The Power of God™, on Gunman, splatting him instantly.

In Duel - The Final Finale, Sensei makes a cameo appearance in the background as Dadman mentions 'a real challenge' for Sonman as he gets his first real sword in the flashback.

Abilities
Time Stopping, Possessing Entities, and Dueling

Trivia
'Sensei' might be a reference to martial arts trainers with the same name.

Time stopping is a recurring ability in multiple other animes like Jojo's Bizzare Adventure.

Some have theorized that Sensei was in fact Berd, who defeated God and ascended mortal bounds.