The endlessly endearing Mr. Mub Mub, as voiced by the irrepressibly energetic @baronharkonnen (SpeakerD)
I love how genuinely alien the Xenians and other aliens are in half-life, from their biology to their mannerisms to their speech quirks. SpeakerD does a fantastic job of conveying the true alien grasp of social interactions in a way that is both funny and very empathetic.
The species in HL also have some interesting means of making art too, which is what I tried to get at here. I like to think if Mubby wanted to make a self portrait, it would look a little like this.