I don't know if someone already said this but I feel like the person Goo Ryun killed that Park Joong Gil loved…
I think the reason why she killed herself was because of the grief she felt after her son passed. So I also do believe that Goo Ryun was the person Joong Gil loved.
https://tapcomics.com/webtoons/tomorrow-why-does-lim-ryung-goo-constantly-pick-his-nose/