Feldt is Ruido and Marlene's daughter. Chall was saved by both Ruido and Marlene but they died in process. Hence, the obligation for Chall to protect Feldt at all cost. Plus the fact that young Chall had a one sided crush on Ruido.
12:56 its okay Heero... we understand it's never easy cosplaying as Masaki Andou. Although your personality is polar opposite, your voice is very similar... hehe
Not sure if this has been in a video already but Fon and older Chall have a special interaction when the former supports the latter. Fon: "Chall! Show me how you live your life!" I forgot what Chall responds though or if this also applies to young Chall. They were using Astrea Type F2 and Plutone.
I think it's also because in Gundam EXA's manga like Leos Alloy, Fon became dimension traveler together with Hanayo although he operate separately from Leos. Not to mention both Leos and Fon voiced by Nobuhiko Okamoto.