Heihachi's been a more consistent presence in the Tekken series, though Jin only missed out on Tekken 1 and 2 due to not being created yet. Heihachi's been used in the most crossovers, but Jin's not that far behind. Jin's also been the protagonist longer than any other character.
The only characters in every Tekken title are Heihachi, Yoshimitsu, Nina Williams and Paul Phoenix.
Marshall Law was replaced by his son in T3 and TTT1. Kazuya was missing in T3. Anna Williams was missing in T4. King was replaced completely by King II in T3.
Both show up in Namco X Capcom.
Both PXZ1 and PXZ2.
Both are in Street Fighter X Tekken.
Jin's fighting style and clothing is in Soul Calibur 5.
Heihachi is in Soul Calibur 2.
Heihachi is in Pac-Man Fever
Heihachi is in some Tekken Namco game.
Heihachi was in PSASBR.
Namco was focusing on Jin in Namco's version of Tekken vs Street Figther, unlike Capcom doing so with Kazuya.
Jin's been on the cover of Tekken 3, Tekken 4, Tekken 5, Tekken 5 DR, Tekken 6, Tekken Advance, Tekken 3D Prime and Tekken Tag 2.
Heihachi's been on the cover of Tekken 2, Tekken 4, Tekken 6, Tekken 7, Tekken Tag 2. I forget if he was also on Tag 1's cover.
That's why I see it as the whole Mishima Family being the main characters in Tekken rather than just one.