Was it Mantis?

Not Mantis either; Swordsman brought her into the Avengers.

I'll just toss out a guess and say it was none other then the Iron Man, tho if I recall, it wasn't the real IM who recommended Duquesne, but a creation of the Mandarin.

Your memory serves you but it's mixing up the stories. Back in Avengers #20, the Mandarin used a holographic projection of Iron Man to get the Swordsman into the Avengers as his agent. I'm referring to Avengers #114, wherein the Swordsman reformed for real and re-applied for Avengers membership. Though Iron Man was willing to give him a chance (citing other Avengers with prior criminal records), he was not the Avenger who observed Swordy during his probationary period and later recommended him for full membership.

Hmmm...had to think about who was part of the Avengers roster at that time (I originally thought Hawkeye...but he was part of the Defenders since it was just before the Avengers-Defenders war)...so that leaves the character who was often the voice of reason (and my guess)...Black Panther

An excellent guess, and the Panther did keep Swordy under close scrutiny even after his acceptance; but he wasn't the observing member.

We're running out of options here...I'm pretty sure it was Thor.

And Thor it was indeed. All yours, DZ.

Which Marvel hero once became involved in the apprehension of a serial murderer called the King-Cake Killer?

I don't know, but when he caught him he was the toast of the town.

Boy, it takes a lot of crust to tell that joke.

Don't stop me, I'm on a roll!

Namor, the Avenging Son.

Are you Dani...err... Finished?

Don't blame me, it's just the way I was bread.

Yes... In.

Namor is right...JTR has the floor.