Yabbut, Worm!Leviathan won't embarrass her with all sorts of infantile behavior, or showing everyone those baby photos, or telling everyone and anyone those stories.....
Besides, Sona doesn't look at Serfall and see "Eldritch Abomination" (most of the time), she sees... her used-to-be-cool Big Sis, who seems hell-bent (ahem) on being Misato Katsuragi, with less booze&sex and more Mahou Shojou cosplay.
Also, Worm!Leviathan won't break down in tears of devastated, heartbroken betrayal every time Sona yells at her... er, it. And being able to finally just snap and go all medieval on some Leviathan's ass would probably do Sona some good. That girl is just too repressed.
Besides, Sona doesn't look at Serfall and see "Eldritch Abomination" (most of the time), she sees... her used-to-be-cool Big Sis, who seems hell-bent (ahem) on being Misato Katsuragi, with less booze&sex and more Mahou Shojou cosplay.
Also, Worm!Leviathan won't break down in tears of devastated, heartbroken betrayal every time Sona yells at her... er, it. And being able to finally just snap and go all medieval on some Leviathan's ass would probably do Sona some good. That girl is just too repressed.