Justin Roiland is an American actor, animator, writer, producer and director. He is best known as the co-creator and executive producer of Adult Swim's Rick and Morty, in which he voices the show's titular characters. He also voices Oscar on Disney Channel's Fish Hooks, the Earl of Lemongrab on Cartoon Network's Adventure Time and Blendin Blandin on Gravity Falls. Description above from the Wikipedia article Kari Wahlgren, licensed under CC-BY-SA, full list of contributors on Wikipedia.

Justin Roiland plays as Rick Sanchez, Morty Smith (voice) in Solar Opposites.
Justin Roiland is the top #1 member of the cast and appears in 41 episodes.

Sean Giambrone (born May 30, 1999) is an American actor. His roles include Adam F. Goldberg in the sitcom The Goldbergs, and Ron Stoppable in the Disney Channel live action version of Kim Possible. He was the voice of Jeff Randell in Clarence, and Russell in Russell Madness. He voices Yumyulack in Solar Opposites. He also had a small role in The Emoji Movie.

Sean Giambrone plays as Ben Pincus (voice) in Solar Opposites.
Sean Giambrone is the top #2 member of the cast and appears in 29 episodes.

A hybrid of Minnesota and Wisconsin (with a severe cheese dependency), comedian Mary Mack is a favorite on radio shows and podcasts around the country including The Bob and Tom Show, Marc Maron's WTF Podcast, and The Grand Ole Opry. Mary played the leading character, Dylan, on Fox’s cartoon Golan the Insatiable, season one. This year, she plays a lead character in the Hulu cartoon Solar Opposites by the creators of Rick and Morty. She’s appeared in Montreal’s Just For Laughs Festival, HBO’s Andy Kaufman Awards, San Francisco’s Sketchfest, and the Dry Bar comedy series. Mack’s television credits include Comedy Central's Live at Gotham, TBS’ Conan, Adult Swim’s Aqua Teen Hunger Force and NBC’s Last Comic Standing and Last Call with Carson Daly. When not touring, Mary spends way too much time and money working on her vintage camper in the woods of Northern Wisconsin.

Mary Mack plays as Jesse (voice) in Solar Opposites.
Mary Mack is the top #3 member of the cast and appears in 8 episodes.

Thomas Steven Middleditch is a Canadian actor, comedian, and television writer, known for his role as Richard Hendricks in the HBO series Silicon Valley, for which he was nominated for a Primetime Emmy Award for Outstanding Lead Actor in a Comedy Series. Wikipedia

Thomas Middleditch plays as Drew in Solar Opposites.
Thomas Middleditch is the top #4 member of the cast and appears in 6 episodes.