I’m not actually having a kid, I’m just trying to make a character and I don’t know any nicknames that would be suitable for [name_f]Alethea[/name_f]. Any suggestions?
[name_u]Ali[/name_u] and [name_f]Thea[/name_f] are the most obvious ones that jump out at me.
Some others that might work:
Lethee
[name_m]Al[/name_m]
[name_f]Letha[/name_f]
[name_u]Lee[/name_u]
I think [name_f]Lettie[/name_f] or [name_f]Leafy[/name_f] could be sweet, since “th” sounds are traditionally replaced with other similar consonants in diminutives (think [name_f]Betty[/name_f] or [name_f]Buffy[/name_f] for [name_f]Elizabeth[/name_f]).
I really like the suggestion of [name_f]Leafy[/name_f].
[name_u]Ali[/name_u]
[name_f]Alya[/name_f]
[name_f]Thea[/name_f]
[name_f]Lea[/name_f]
[name_f]Leta[/name_f]
[name_f]Lettie[/name_f]
I love this name, btw! 
[name_f]Alethea[/name_f] nicknames:
[name_f]Leta[/name_f]
[name_f]Lethia[/name_f]
[name_u]Allie[/name_u]
Als
[name_f]Thea[/name_f]
[name_f]Alia[/name_f]/[name_f]Alea[/name_f]
[name_u]Lee[/name_u]/[name_u]Leigh[/name_u]