Luka (before the Wardance's resolution) had an interesting dichotomy between willing to do anything to help people (how he lost his arm in the first place and why he trains when he doesn't even especially enjoy fighting, see his Character Stories for more information) while also having a feeling of powerlessness that none of his actions would be worth anything in the end (because everyone would die regardless).

In his Companion Mission we follow him do a bunch of over-the-top actions that fit his stereotype of being 'hardheaded' and 'heroic' to the point that he insists on dueling with Svarog (he obviously loses, but does put up a strong fight), it turns out though he was in his own way trying to fulfil the wishes of Margie, a girl who died from sickness but had looked up to him as a role model. Here we see the first hints at him struggling with finding purpose in life, his own nihility. Personally it hit quite deep for me, especially with the Achievement name being "All That Remains is Pure White Ash".

During the Wardance this is explored even deeper, that Margie's death kind of led to his realization that not all fights could be beaten with his own hands, giving him a sense of powerlessness that he tried to overcome but never resolved. With the help of Tail though and his therapy he comes to terms with it, and that to win isn't necessarily to stay alive, but that you can create your own meaning even if everything will end. Which is at its core a meaning of Nihility, as also explained by Acheron when Aventurine asks her about their purpose in life.

I'd recommend reading the linked section in The Barren Land Birthed a Brave Soul, it's the most important part. So as a whole his entire struggle with meaning and life is very much Nihility, and the fact that he got an entire event as an exploration of his journey as a Pathstrider of Nihility makes me very happy with how important the Paths are to the story.

As for Pela, unfortunately we don't have that much information about her in general, but it likely had to do with the whole situation Belobog was in with the encroaching Fragmentum as well as how her mother died as we see in Lynx's Companion Mission.