Suggestions by @jemmaross

Siren Noises

as Siren Head in Siren Head

Jay Leno

as Jay Leno in Siren Head

Winston Duke

as Matt Thomason in Siren Head

Joaquin Phoenix

as J.J Paulson in Skibidi Dom Dom Yes Yes Toilet

Amir Talai

as Huandor Skavidi Skavidbop in Skibidi Dom Dom Yes Yes Toilet

William Baldwin

as Arthur Camshaft in Skibidi Dom Dom Yes Yes Toilet

Millie (Helluva Boss)

as Lilith in World of mirrors

Moxxie (Helluva Boss)

as Thomas in World of mirrors

Blitzo (Helluva Boss)

as Detective Mack in World of mirrors

Shin Godzilla

as Chief Cerris in World of mirrors

Judge Doom

as The Waiter in World of mirrors

Cujo

as The Greeter in World of mirrors

Great Mighty Poo

as The Follower in World of mirrors

Chazwick Thurman (Helluva Boss)

as Dazer in World of mirrors

Crimson (Helluva Boss)

as Boss in World of mirrors

Zac Efron

as Vivirous in Funhouse Hotel

Ego Plum

as Composer in Funhouse Hotel

Fifty Fifty

as Composer in Funhouse Hotel

Eric Stoltz

as Writer in Funhouse Hotel

Chris Savino

as Producer in Funhouse Hotel