Adding fresh faces to a long‑running series is a tightrope act, and the phenomenon of late‑stage additions has even earned its own name: the Cousin Oliver trope. While many newcomers get the short end of the stick, some become instant legends. In this roundup we’ll count down the ten most memorable 10 popular tv characters who arrived after the pilot and left a lasting mark on their shows.
Why Late Arrivals Matter for 10 Popular TV Series
From comic relief to dark anti‑heroes, the characters on this list proved that a well‑timed injection of new blood can rescue a sagging plot, deepen world‑building, and even pull a series back from the brink of cancellation. Let’s dive into each of these fan‑fav additions, complete with the back‑story of how they slipped onto the screen.
10 Frank Reynolds: It’s Always Sunny in Philadelphia
The long‑running FX comedy It’s Always Sunny in Philadelphia started as a low‑budget experiment by Charlie Day, Glenn Howerton and Rob McElhenney, who filmed their own pilot on a Panasonic DVX100A. After the pilot was green‑lit, the show struggled to find an audience, prompting executives to wonder what was missing from the ensemble.
Enter Danny DeVito. The seasoned actor, known for his affable public persona, was cast as the scheming stepfather Frank Reynolds in season two. DeVito’s presence added a layer of crass, unapologetic cynicism that perfectly complemented the existing dynamic.
Despite initial reservations from the original trio, DeVito’s character quickly became the show’s dark heart, delivering some of its most outrageous moments. His addition is widely credited with pulling the series out of a near‑cancellation scenario.
Fans today agree that Frank Reynolds was the missing piece that turned a promising comedy into a cultural touchstone, cementing the show’s place among the longest‑running sitcoms on television.
9 Ben Linus: Lost
When ABC launched Lost, the mystery‑laden island drama captivated viewers from day one. The series’ second season introduced a new player who would become one of its most compelling antagonists: Benjamin Linus, portrayed by Michael Emerson.
Ben first appears as a captive labeled “Henry Gale,” misleading the survivors into believing he’s an ordinary man. By the season’s climax, the façade shatters, revealing him as the mastermind behind the enigmatic Others. His morally gray tactics, relentless quest for power, and occasional acts of brutality quickly made him a fan‑favorite.
8 Fin Tutuola: Law & Order: Special Victims Unit
Ice‑T, already a household name in music, transitioned to television at the start of SVU’s second season, taking on the role of Odafin “Fin” Tutuola. A former narcotics detective, Fin brings a street‑wise edge to the squad while harboring a deep commitment to protecting victims of sexual assault.
Paired with the conspiracy‑loving Detective Munch, Fin’s tough exterior and unwavering compassion created an instant chemistry that resonated with audiences. Over 22 seasons, he has become the longest‑tenured non‑original cast member, eventually rising to captain‑in‑training and serving as the right hand to Olivia Benson after Elliot Stabler’s departure.
7 Rafael Barba: Law & Order: Special Victims Unit
The assistant district attorney seat on SVU has seen many faces, but none have left as strong an imprint as Rafael Barba, brought to life by Broadway veteran Raul Esparza. Barba debuted in season 14’s “Twenty‑Five Acts,” instantly standing out for his razor‑sharp legal mind and impeccably tailored suits.
Elevated to series regular the following season, Barba’s blend of wit, sass, and strategic brilliance made him the first male ADA to anchor the main cast. Though he exited in 2018 to return to the stage, his divisive farewell in “The Undiscovered Country” still sparks discussion, and he continues to pop up as a guest star in later seasons.
6 Desmond Hume: Lost
Desmond Hume entered the Lost mythos with a bang, surfacing in the season‑two opener as the lone occupant of the enigmatic hatch. Though his early screen time was brief, the character’s solitary confinement set the stage for one of the series’ most iconic arcs.
After a long hiatus, Desmond reappears in the season‑two finale and becomes a regular in season three. His journey—from a man convinced the world outside the island no longer exists to a lover torn between timelines—culminates in the beloved episode “The Constant,” often cited as the series’ highest‑rated installment on IMDb.
5 Tommy Oliver aka the Green/White Ranger: Power Rangers
The inaugural season of Mighty Morphin Power Rangers thrilled 1990s kids with its five‑color team, but the 17th episode introduced a game‑changing newcomer: Tommy Oliver, initially under Rita Repulsa’s spell. Played by actor‑martial‑artist Jason David Frank, Tommy quickly became the series’ breakout star.
Fans connected with his quest for identity and belonging, and the writers crafted a redemption arc that saw him transition from the villainous Green Ranger to the heroic White Ranger. His romance with Pink Ranger Kimberly added another layer of fan‑service, cementing his status as one of the franchise’s most enduring characters.
4 Michonne: The Walking Dead
Katana‑wielding Michonne Hawthorne first slipped onto The Walking Dead in a brief cameo at the end of season two, but she didn’t become a mainstay until the following season. Portrayed by Danai Gurira, Michonne arrived alongside Andrea before striking out on her own.
After parting ways with Andrea, Michonne crossed paths with Rick Grimes and his core group. Though initially guarded, she forged a close bond with Carl and eventually entered a complex, often tumultuous relationship with Rick himself.
Michonne remained a principal character through ten seasons, amassing a kill count that rivals the series’ most lethal survivors and becoming a fan‑favorite for both her combat prowess and emotional depth.
3 Lexa: The 100
The CW’s post‑apocalyptic drama The 100 introduced a pivotal figure in its second season: Lexa, a fierce leader of the Grounders played by Alycia Debnam‑Carey. Initially presented as a limping servant, Lexa’s true identity as the Grounders’ commander was revealed early in the season.
Her dynamic with series protagonist Clarke quickly evolved into a groundbreaking romance, making Lexa one of the show’s most iconic characters. However, in season three, a stray bullet meant for Clarke claimed Lexa’s life, sparking intense fan backlash and a noticeable dip in viewership.
Off‑screen, Debnam‑Carey’s commitment to a new role on AMC’s Fear the Walking Dead forced her departure, and while the series soldiered on for four more seasons, it never fully recaptured the cultural momentum Lexa had generated.
2 Spike: Buffy the Vampire Slayer
Joss Whedon’s cult classic Buffy the Vampire Slayer welcomed a charismatic vampire named Spike in the third episode of its second season. Played by James Marsters, Spike’s bleach‑blond hair, leather jacket, and motorcycle swagger made him an instant anti‑hero.
While Angel embodied the tormented vampire with a soul, Spike reveled in his unapologetic bad‑boy persona. Yet beneath the surface, he harbored a surprisingly tender side, often expressing his love for poetry and romance, especially in his contentious relationship with Buffy.
Spike’s tenure spanned six seasons on Buffy and extended into the spin‑off Angel, where his evolution from villain to complex ally kept fans debating his moral compass well beyond the series’ conclusion.
1 Klaus Michaelson: The Vampire Diaries
The CW’s teen‑focused supernatural drama The Vampire Diaries took a bold turn in its sophomore season by spotlighting the Original vampire family, led by the enigmatic Klaus Michaelson. Portrayed by Joseph Morgan, Klaus is the patriarch of the Originals, a lineage of vampires that can only be slain with a stake forged from a rare, ancient tree.
What sets Klaus apart is his hybrid nature—part vampire, part werewolf—making him the series’ first true hybrid. His formidable strength, strategic mind, and tortured artistic soul resonated with audiences, elevating him beyond a typical villain.
Klaus’s popularity sparked a dedicated spin‑off, The Originals, where his complex family dynamics and relentless pursuit of power continued to captivate viewers, solidifying his legacy as one of television’s most memorable late‑stage introductions.