Medusa was first introduced in FF #36 in a flashback told by the Wizard as he muses over how fortunate it was that Paste Pot Pete (soon, the Trapster) and Sandman stole a jet plane after breaking out of prison and snagged him from floating away in the upper atmosphere. All three hated Johnny Storm, and something one of them says triggers a thought, a memory from the Wizard that if only they had a fourth, female member, they could be the equal of the FF...an evil FF. The Frightful Four...
He thinks he has just the candidate, a wild, ferocious woman wearing tattered buckskin dress* that he witnessed make short work of a team of French policemen. It was Medusa, hiding from the law. Her long live flowing tress whipped the police in short order. He says she was magnificent. He's sure he could convince her to join them.
Flash forward to a few days later as he introduces Madam Medusa in a violet and purple outfit to Pete and the Sandman. She asks "why does HE not have a costume" of the Sandman, who makes a play for her saying, "I don't NEED one, baby. Wait til you see me in action, Then you'll know why." hehehe... The Frightful Four assemble for the first time. (Note the bizarre French headdress she wears this time only. It will be almost 30 years later before DC tries to re-invent the Creeper as a french woman that we see this style of headress again!)
The Evil FF set up the good FF during the planning for Reed and Sue's wedding, and Medusa proves to be a cunning and possibly deadly adversary. The Evil FF escape and return, beating the FF through divide and conquer time and time again, and in #38 they lure the FF to a pacific atoll where an A-bomb has been left behind, and strand the FF there to take the impact of the bomb (similar threat used in #28 with the X-men guest-starring)...and they figure they're gone when the bomb goes off. But they hadn't reckoned with Sue's force field.
The evil FF are cautious in their return to the Baxter Building to loot it, and in a clever cross-over with Thor's adventure, they mistake Balder's flaming arrival on earth for evidence that the Torch has survived the A-bomb, and flee. But a short time later after a pitched battle with Dr. Doom on the streets of manhattan, they are in hidding in the suburbs. and when they lure Ben away from the FF and use the Wizard's I.D. machine (hither to unknown or unmentioned) to try to tinker with his mind and turn him against the FF, a battle royal ensues. Johnny is captured and also subjected to the I.D. machine. However, unknown to all, Sue had turned invisible and sabotaged the machine...Johnny's faking his agressive allegence to them.
Reed and Sue kidnap a fighting mad Thing from the Evil FF and take him home to the HQ, where Reed trys to reset Ben's mind. The Evil FF show up with Johnny held hostage, and demand their surrender (for what purpose?) and Reed stalls for time as Ben wakes up. In one of the most dramatic turn-arounds of all time, Ben grabs the Wizard by the chest and crushes his suit, freeing Johnny as well, and the evil FF are rounded up...all except Medusa, who Johnny "lets" escape, as he starts daydreaming/fantasizing about her as she runs into the ship to fly off. End of the 3 part story arc as Reed and Sue are about to get married.
Medusa, on the run from someone named "Gorgan", stows away in Johnny's car at Empire U campus and the tale of the Inhuman's arc begins. Medusa is painted sympathetically "on the run", fearing Gorgan who is hunting her, and Gorgan succeeds in kidnapping her and flying away in the FF's helicopter (guess he knows how to fly stuff). From there out, Medusa is painted as a quasi-heroic misunderstood member of the royal family, all on the run from The Keeper...sent by Maximus the Mad to collect the royal family... and the rest is history.
*No, Medusa was shown in flashback as a "hardened criminal" in clothes much like Rachael Welch wore in 2 Million Years B.C. or Ursula Andrews in Dr. No. There is NO indication she is anything but a criminal on the run on some unnamed Mediterranean Island (near France?) at first...but as this is a flashback told by the Wizard to his rescuers, who he is scheming to dominate and lead, who knows. (My theory is that he had to find her and subjugate her to his will with the I.D. Machine.)
The concept of the Inhumans is not introduced until #45, about 9 months later, after multiple appearances as a wicked threat and temptress who enjoys playing men against each other. Jack and Stan retooled her on the fly when the response to rumors of Joe Simon starting a superhero line up fell through.
She's had a varied path since that first year, but usually as interpreter for Black Bolt, wife of the king, sister of Crystal, and more than once, substitute team member in the FF, and fast friend of the Richard's/Storm family. More than once, the evil FF have attempted to manipulate her back into their fold, to their regret and dismay, she's foiled their plans. (Marvel Super-Heroes #15 and others)