Sol

Appearance
Sol is an orange male wolf.

Personality
Sol took after his mother and likes to explore far from the den, often away from the other pups. Being further away from the other wandering wolves makes it harder to keep him at the den, and also allows him to annoy Gold and Cloud, which he seemed to enjoy.

Gold
Sol takes after Gold in coloring and personality, but they don't get along well, as Gold has to regularly scold him for wandering off.

Cloud
Cloud also gets annoyed at Sol for wandering off but isn't as harsh in his scolding. Sol and Cloud don't pay too much attention to one another unless Cloud has to carry Sol back to the den, but otherwise they passively co-exist with one another.

Copper
Sol and Copper usually split up when exploring and go in opposite directions to let one pup get further away from the den while the other pup is "rescued" (who needs rescuing?) by Cloud or Gold. However, despite this, Copper made no move to warn Sol or to tell Gold or Cloud when a coyote was tailing Sol and when a coyote (presumably) killed Sol later on.

Slate
Slate and Sol didn't hang out much for the short time that Sol was alive. They play-fought a few times but beyond that never had a good family relationship. They may get along better now in death, as Slate did feel guilty for not trying to make a better relationship with her brother when Sol died.

Coal
Coal and Sol often argued, but still were as close as brothers could be. Coal normally said that they shouldn't leave the den as they may get eaten by a coyote or something, but followed him anyways. Despite this, he didn't seem to notice when Sol was being tailed by a coyote.

Death
Sol was presumably killed by a coyote, as one was tailing him earlier in the episode, but Gold wasn't within sight when he died.

Trivia

 * "Sol" means "Sun" in Spanish and Latin.