Proteus and Kiril are awesome. Even very advanced players with several fully leveled 5* heroes still use them a lot (unless they have Hel which is more or less a 5* version of Proteus)
Li Xiu is also great and I still use her a lot, in particular against teams with a healer tank to keep the mana of the other opponents low while I fire tons of tiles against the healer in the middle. Not an absolute must have like Wu Kong, but slightly better than Chao and far better than that 4* panda.
Kelile is a good sniper in the early and mid stage of the game - however, there are much better red 4* heroes (Boldtusk in my opinion the best of the trainable ones). In later stages of the game, when you have 5* heroes and the materials to pay for the last ascension level, Kelile will be replaced by much stronger snipers.
I would leave her at 3/60 and wait for a really great red 4*.
Kasshrek, however, is only a decent tank but neither very useful nor fun to play in PvP offense or map levels. I even regret having him brought to 3/60 - should have worked on my second Caedmon before