The Fantastic Four kicked off the modern age of Marvel Comics with the very first issue of their self-titled comic in 1961. Since then, "Marvel's First Family" has gone on to become one of the pillars of the Marvel Universe, introducing many iconic characters and concepts.
While the classic lineup, consisting of Mr. Fantastic, Invisible Woman, Human Torch, and the Thing, is the most well-known and frequent roster, they are not the only characters who have joined the team. The originals have at times taken sabbaticals, prompting others to step up as alternates.
Luke Cage
Hero For Hire Luke Cage was contracted to replace the Thing during a period in which Ben had temporarily lost his powers. While he was personally selected by Reed, and bonded with Sue during his short tenure, Cage was resented by the outgoing Ben, who had been previously unaware of a clause in the Fantastic Four's team charter stating that all members must possess superpowers. Cage only lasted for two issues before the villain Puppet Master took control of him, forcing him to battle the others. To combat Cage, Reed built a Thing-like exo-suit for Ben, allowing him to rejoin his teammates until he eventually regained his original abilities.
H.E.R.B.I.E.
H.E.R.B.I.E. (Humanoid Experimental Robot B-Type Integrated Electronics) first appeared in the 1978 New Fantastic Four cartoon, as a replacement for the Human Torch, whose rights were tied up at the time in a solo film that ultimately never materialized. H.E.R.B.I.E. was disliked by most fans but eventually made his way into the comics anyway, though in a different capacity.
Comic book H.E.R.B.I.E. instead stood for "Highly Engineered Robot Built for Interdimensional Exploration" and was initially designed to survey for the potential return of Galactus. Other versions were later built to act as housekeepers and lab assistants in the Fantastic Four's Baxter Building headquarters, with one, in particular, acting as the guardian to a young Franklin Richards.
Frankie Raye
After an accident involving chemicals developed by her stepfather, Frankie Raye developed powers almost identical to those of Johnny Storm, the Human Torch. Unbeknownst to Raye, her stepfather was secretly Phineas Horton, the creator of the original android Human Torch, from which Johnny had adopted his legacy.
After falling in love with Johnny, Frankie would begin frequently joining the Fantastic Four's adventures, becoming increasingly more proficient in her abilities. She was eventually invited to officially join the team, with the originals even considering changing their name to "The Fantastic Five". Unfortunately, shortly after being offered membership, Frankie would volunteer herself to become the Herald of Galactus in order to spare the Earth from the eater of worlds.
Sharon Ventura
An on-and-off girlfriend of Benjamin Grimm, Sharon Ventura is best known to fans as She-Thing, although in the comics she has generally preferred the name Ms. Marvel, despite others frequently using the moniker. After multiple team-ups with Ben, she was invited to join the Fantastic Four as a replacement member when Reed and Sue Richards were offered Avengers membership.
Initially possessing superhuman strength and durability, but retaining her human appearance, an encounter with cosmic radiation resulted in her transformation into a rocky form similar to the Thing's. While she continued to adventure with the team, following the return of Reed and Sue, Ventura became disillusioned with Reed's inability to cure her condition. She eventually betrayed the Fantastic Four and aligned herself with Doctor Doom when he promised he could do what "That Fool Richards" could not.
Medusa
Medusa began as an enemy of the Fantastic Four, but she and her fellow Inhumans gradually became trusted allies. After years of frequent team-ups, Medusa was asked to replace a recently departed Invisible Woman. Sue had quit the team and separated from Reed after a series of increasingly heated arguments about her role following the birth of their son, Franklin.
While Medusa is an interesting character and her inclusion in the Fantastic Four brought with it a new and exciting dynamic, she was hard to cheer for due to the manner in which Sue had vacated her position. Despite this, Medusa lasted an impressive 27 issues, from #'s 132-159, and redeemed herself by convincing Reed to reconcile with his estranged wife, voluntarily stepping down when Sue rejoined.
Ant-Man
When the Fantastic Four's resident genius Reed Richards seemingly perished alongside arch-nemesis Doctor Doom, the team found itself left with a significant void. To fill the missing role of the group's technical expert, Scott Lang, better known as the hero Ant-Man was hired at the recommendation of Iron Man. Lang was excited by the opportunity to join the team of celebrity adventurers because his daughter Cassie, the future Young Avenger Stature, was a fan.
Although he has never been the super-genius that Reed is, Lang's intelligence and technical expertise proved an asset, even as he found himself frequently out of his depth during the team's often unconventional adventures. Scott lasted with the team from issues #384-408, stepping down when Reed rejoined after being found lost in time.
Storm and Black Panther
Tenured X-Men leader Storm and her husband, Wakandan King Black Panther, were handpicked to join the Fantastic Four following the superhero Civil War. Reed and Susan Richards had aligned with opposing sides during the conflict, and, after reconciling, opted to temporarily retire from super-heroics to focus on their strained marriage.
While Storm's raw power and Black Panther's intelligence were more than appropriate replacements for the Invisible Woman and Mr. Fantastic respectively, their tenure with the team was short and marked by conflicts of responsibility. While adventuring alongside original members Human Torch and The Thing, Storm was still an active member of the X-Men and T'Challa the ruling King of an entire country. These previous dedications unfortunately often diverted the couples' attention from their newly assumed roles.
She-Hulk
Possibly the most well-known of all of the alternate members of the Fantastic Four, Jennifer Walters, the Sensational She-Hulk, joined following the Thing's decision to remain behind after Marvel's first-ever crossover event, the Secret Wars. A suitable replacement for the missing strength of Ben Grimm, Jen also brought some much-needed levity to the team's often offbeat and otherworldly exploits.
During her time with Marvel's First Family, Walters also began a relationship with frequent Fantastic Four ally Wyatt Wingfoot, which remains as an occasional romance in the comics to this day. Jen served admirably during her time with the team, lasting an impressive 35 issue, three-year-long run, from #'s 265-300, and only stepping down when Ben Grimm finally returned home.
Spider-Man
If ever there were a character who deserved membership amongst the exclusive ranks of the Fantastic Four, few could argue against Peter Parker, the Amazing Spider-Man. In fact, in the very first issue of his ongoing series, Spider-Man attempted to break into the Baxter Building to prove himself worthy of joining their ranks. While he was denied, this kicked off a long tradition of team-ups and established his ongoing friendship with the Human Torch, which has become one of the most iconic and unexpected friendships in Marvel Comics.
Unfortunately, Spider-Man's long overdue invitation came in the midst of less than desirable circumstances. Following the Human Torch's apparent death during a mission to the Negative Zone, Peter was offered membership as per Johnny's final wishes. Despite the sorrowful circumstances, Spider-Man threw himself into his work with the team, more than living up to the legacy of his fallen friend.
Crystal
Crystal, the younger sister of Medusa, is likewise a part of the Inhuman Royal Family. When the Inhumans and the Fantastic Four began a series of misadventures together, Crystal and Johnny Storm were immediately smitten with one another. Following this long-term long-distance relationship, Crystal was recruited to replace the Invisible Woman during Sue's first pregnancy. She would later be called back to join the team alongside Sharon Ventura when Reed and Sue were drafted to The Avengers.
By this time she and Johnny had ended their relationship and Crystal had married, then separated from the Avenger Quicksilver, bearing a child along the way. Although Crystal's first tenure with the Fantastic Four was defined by her wide-eyed inexperience and reliance on Johnny, her second stint saw a more focused and mature Crystal who easily established herself as the team's most powerful member.