Elio pulls a Nero and says "Umu" when he's ecstatic or pleased about something.

It's a verbal tic he inadvertently developed post-emotional discovery, partly rooted in his usual and childish responses to his older brother Ardyn when he was a toddler: a one world gibberish response («Ungh», «Ah», «Heh», etc.) or him rapidly shaking his head in certain directions.