characters in animation we can't understand but the characters can.

[hitch4645]

It's Phineas and Ferb.​

 

[Pizza Lover]

And it doesn't count because Ferb does speak. Sure, he only talks about once or twice per short, but when he does speak, he does so intelligibly and the audience can understand him.

 

[Sloth]

Ferb is basically Silent Bob to Phineas' Jay. He CAN talk, but only does so when he has something to say.

 

[slman619]

boomhaur

 

[jennifer_goble]

On Frisky Dingo, only Killface seems to be able to understand his son most of the time, although Simon occasionally has intelligible lines, including the last line of Season 1:
"Heheheheheh, BOOSH!"

 

[Kunmui]

Let's not forget Buzzbomb (C.O.P.S.) the robot who makes all sorts of crazy sounds, yet he can only be understood by his creator, the bad butt Dr. Badvibes.

 

[seba7740]

In "Dastardly and Muttley's Flying Machines", the Klunk character always spoke with weird sound effects added in, and only Zilly understood what he said.

 

[Hitaro]

Plank from Ed, Edd n Eddy... but, only Johnny understands him, for the most part.