Director Rob Letterman says first generation Pokemon characters were a must.

Pikachu will return to the big screen, in all his electrifying and "pika-pika" glory, when Detective Pikachu hits theaters May 10. 

The iconic yellow creature (voiced by Ryan Reynolds) from the Pokémon game series joins Ryme City resident Tim Goodman (Justice Smith) on his quest to find his missing father.

Director Rob Letterman spoke to The Hollywood Reporter about how he came to bring the story of Detective Pikachu to the big screen, explaining that while he was initially unfamiliar with the world of Pokemon, his two children are devoted fans and inspired him to take on the project.

"My credit card was very familiar with Pokemon, then I was very familiar with it -- Pokemon cards, the toys, the video games," he joked. "It was in our household for a while even before I heard about the movie." 

The film, which primarily takes from the 2017 video game of the same name, holds a "really human story" with a "really fabulous character" that Letterman said Reynolds perfected. 

Reynolds' voice exudes the big personality necessary to make this film's main character pop, Letterman said. He said when he matched the audition animation with Reynolds' voice, it seemed perfect. 

"The second I saw that, it was a done deal. That was it, it was perfect," he said. "Sometimes things just sit perfectly."

On selecting which Pokemon to include in the film, Letterman said he worked with designers and the Pokemon company. Though the Ryme City's population comprises of Pokemon from different generations, Letterman said first generation characters were a must. 

"The one thing I discovered quickly was that the fans aren't just kids," he said. "There's fans on the crew in their 20s and early 30s, so we  picked a lot of 1st generation Pokemon so they can connect with this and relive their childhood."

From Charizard to Greninja (and Squirrel, Cubone, Gyrados and Mr. Mime), here's what to know about the different Pokémon in Detective Pikachu. 

• Pikachu

  

  One of the most well-known characters of the Pokemon franchise, Pikachu is a small creature with yellow fur and a lightning bolt for a tail. 

  An electric-type Pokemon, Pikachu has a number of attacks including electrically charged moves like thunder shock, thunder wave and thunder bolt. As the nature of physics would suggest, Pikachu's weakness is ground, which hinders the impact of his attacks. 

  Though Detective Pikachu puts a live-action twist on the iconic character, it's far from Pikachu's first TV or film appearance. He's starred in a number of titles ranging from Pokemon: Pikachu's Vacation in 1998 to 2017's Pokemon the Movie: I Choose You!, where he speaks to his trainer Ash for the first time ever.

• Psyduck

  Unlike Detective Pikachu's protagonist, Psyduck is a water-type Pokemon. The duck-like creature constantly holds its hands to to its head because of what seems like never-ending headaches, which result in its most powerful attacks. 

  Though Psyduck's psychic attacks are powerful against steel, fire, water and ice-type moves, they falter against grass and electric charges.

  In Detective Pikachu, Psyduck accompanies Tim Goodman and Pikachu on their journey along with his trainer Lucy (Kathryn Newton). The Pokemon relies on calm elevator music and soft sounds to relieve his potentially disastrous headaches. 

• Greninja

  Hailing from Pokemon Generation VI, Greninja will boast its speed and swift attacks in Detective Pikachu.

  Greninja, the final evolution of the Froakie water-type Pokemon, is a water-dark type Pokemon. Known as the Ninja Pokemon, Greninja conjures ninja stars out of compressed water strong enough to cut through metal. Along with its ninja-like abilities, Greninja can also duplicate itself to threaten foes. 
• Aipom

  Purple monkeys don't exist on Earth, but Ryme City is chock-full with them.

  The Aipom is a long-tail Pokemon that typically bares a wide, full-toothed smile. Unlike regular Earth monkeys, this Pokemon has a functional hand at the end its tail that helps it travel and collect different items. In Detective Pikachu, Aipom takes on a less friendly manner and appearance. The Generation II Pokemon, baring its big teeth in anything but a friendly smile, chases Tim Goodman and Pikachu during their investigation.

  In the gaming series, the Aipom transforms into the Ambipom after learning the double hit attack. In this evolution, the monkey-like creature gains two tales with hands at the end of each of them. 

• Snubbull

  Snubbull brings its mean bulldog mug to Detective Pikachu as it follows Detective Yoshida (Ken Watanabe) throughout Ryme City.

  The Generation II character, also known as the Fairy Pokemon, bares its intimidating fanged-underbite to scare away smaller Pokemon. It also finds strength in Fairy-type attacks like headbutt, lick and more but its combat moves fall flat when it comes to steel and poison-type Pokemon. 

  In the games, Snubbull evolves into the bigger, more powerful Granbull when it gains enough experience points. 

• Charizard

  Charizard will bring the heat to Detective Pikachu. Known as the Flame Pokemon, Charizard is one of the more recognizable characters from the TV and gaming franchise. A final evolution of the Charmander and Charmeleon Pokemon, Charizard goes head to head against Pikachu in the seedy battle arena of Ryme City.

  Though Charizard's attacks include wild tackle, slash and continuous blaze ball, its ability to breathe fire is the highlight of its skillset. 
• Ludicolo

  In Detective Pikachu, Ludicolo mans a Ryme City coffee shop and supplies the titular character with coffee that's "black as night."

  The water-and-grass type creature, commonly known as the carefree Pokemon, breaks into dance anytime it hears festive music, even in time of peril. Though Ludicolo has a roster of dance-based attacks including circular steps and splash dance, it's weak against attacks from bug, flying and poison-type Pokemon. 

  This Pokemon first appeared in Generation III of the Pokemon franchise. 

• Jigglypuff

  Though Jigglypuff seems to be a soft, pink ball that sprouted hands and feet, there's more grit and danger to the Pokemon than meets the eye. 

  Throughout the entire Pokemon franchise the Generation I creature attracts its attackers with its large blue eyes before serenading them into at sleep-inducing trance with sonorous lullabies. 

  In Detective Pikachu the balloon Pokemon brings its snooze-inducing powers to the big-screen, wielding its signature microphone and singing those around it into a deep sleep. 

• Bulbasaur

  Bulbasaur will join the rest of its Generation I Pokemon friends in theaters when Detective Pikachu releases on May 10. The seed Pokemon is one of the many creatures that will help Tim Goodman and Detective Pikachu uncover the truth behind the suspicious series of events transpiring in Ryme City. 

  A grass-and-poison type Pokemon, Bulbasaur is known for the powerful vines behind its back that tackle enemies and pick up various objects. In the series, Bulbasaur also shares a close friendship with Squirtle, a water-type Pokemon. 

  After gaining enough experience, Bulbasaur evolves into an Ivysaur, then a Venusaur, with the flower bud growing and then opening up into a small tree as the evolutions progress.
• Lickitung

  It’s pink, it’s short but it’s not Jigglypuff.

  Lickitung, named for it six-foot long tongue, will also join the band of Detective Pikachu, Tim Goodman and others. The licking Pokemon, after learning the combat move rollout, transforms into Lickilicky, a larger version of itself. With paralyzing-licks and saliva, LIckitung poses a threat to most fighting-type Pokemon.