Help with 2 questionable IV heroes

just give female Kana Dark/Light Breath and windsweep (RIP free joshua but this is optional) and have her serve as a buff/debuffer. Otherwise, make her a tank with her default weapon. if shes a debuffer, refine for spd, if tank then def.

Sonya's speed isn't all that great for an offensive build anyway unless you have a boon and maybe change her weapon and refine it for speed or even run brazen atk/spd. most people run green tomebreaker and that works absolutely well. just give her the QR or distant def sacred seal and she can handle blue and some red tome units decently.

i run an offensive set and i constantly rely on blessings and team support to get her over 40 spd with darting blow

Don't know if this is good, but:

Kana (F) -atk +spd
Water breath +spd or +def
Reposition
Aether or ignis
Steady breath
Wrath
Atk smoke
Deflect magic, or QR if you have team support to block a double and want to ruin Ayra's day with an ignis

Hello, your Sonia is not so bad ! She’s not fast ( 31 speed in neutral ) and she’s very resistant ! ( 32 res in neutral ) with atk/res bond + quick riposte + New moon as she is slow, she will fight twice so at the last blow she will be able to activate New moon and with dark Excalibur it’s fatal :3

Kana’s unsaveable, her Atk at neutral is already at jeapordy.

Sonya could work; her speed is kinda bad anyway. You could build her as a mage tank with double Distant Defense, Quick Riposte and Gronnowl for even more res.