Gerry Conway and Steve Skeates script the Codename: Assassin intro in 1st Issue Special 11 (Feb. 76), while Frank Redondo and Al Milgrom provide the art. Oddly, for a character with that name, Codename; Assassin is a hero, not a villain, and one who goes out of his way to use tranquilizer darts instead of bullets in his gun.
At the top of the story we see how he can use his telekinetic ability to walk on air as he bursts in on a mob boss, sedating him in order to raid his files for information on the big boss. He has a miniature blowtorch, designed by an "associate" that we learn nothing about.
But Codename: Assassin does kill before the story gets too far in. When the boss tries to kill him, Assassin emits a blast of lethal mental energy. Now THAT is impressive.
The big boss has been dealing with this guy for a while, and so hires two men with special powers, the elusive Snake, and electricity-channelling Powerhouse.
We find out a fair bit about Codename: Assassin's backstory. His real name is Jonathan Drew. He and his elder sister were orphans, and Drew underwent some experiments to learn about his degree of mental powers. The machine exploded, and boosted Drew's telekinesis to the point where he can walk on air and kill with a thought.
We also learn that it was the mob who killed his sister. He took out the hit men immediately, with a mental blast, but now has adopted his costumed identity to get to the big men behind it.
The story ends with Codename: Assassin facing off against Snake and Powerhouse. But nothing more is heard from him for over 20 years. In James Robinson's Starman series we find out that Codename: Assassin died fairly quickly, possibly even in this fight. A different character using the name and costume, but definitely a bad guy, appeared during Robinson's run on Superman.