Honestly, as cliched as it may be, this development is hardly surprising if one thinks about it with half a braincell. For all we know Souichi got shat on his whole life (iirc it was mentioned that even before his mom turned into a fat unresponsive goblin his father was abusive), and Chika is perhaps the first person to show him some much needed kindness with no strings attached (now if the haters see this as her being unfaithful to Hirota well, to each their own...).
That said, I hope he's aware he's biting far more than he can chew. Last time Tamaki bothered to show a familiar their place Hirota was left puking his ribcage, so good luck.