Lilia uses 良い (yoi) in a way that is not common in everyday life, kind of archaic/formal/classical.
Literally it means “good” but is very adaptable in context, and Lilia uses it to give commands, give affirmation, etc., to make what he says sound older sounding, or theatrical.
Lilia also often repeats わい (wai), another way his phrasing is being made to sound old-fashion/theatrical/whimsical.
It is generally associated with older/rural/eccentric male characters in fictional stories and isn’t a word with a translatable definition, acting as a sentence-ending particle with a little drama/humor/charm, playing into Lilia being mischievousness and theatrical, old fashioned and an older mentor.
Lilia will also often incorporate じゃ (ja) into his speech, which is also associated with older male characters in fiction, like Samurai-era or fantasy settings.
Lilia uses it instead of だ (da), which has been explained more here ->
When Lilia’s voice actor Midorikawa Hikaru met with Silver’s voice actor and congratulated him on the recent birth of his child, he tweeted using Lilia’s emoji 🦇 and Lilia’s use of “ja”.

