Truth's relation with Kalista
As far as Truth's opinion and thought about Kalista, this will cover how he feels about her.
In his mind, Kalista is manifestation of people's cry for revenge. She is a mass of pure anger, of grief and of cold resolve. She is trying to see that personal justice is done.
But this is where the term "Justice is blind" comes into play. To Truth, Kalista heeds and follows the request of whoever does her ritual. That means she may kill people who only acted because they had no other options. If a murderer or a crime boss or even a rapist wanted revenge, Kalista would probably get them their revenge if they were willing to sacrifice their soul. Because for Truth, Kalista's revenge is just selective murder.
As for who she is, Truth understands why she does this. He's been betrayed and has lost loved ones because of it. He understands her want for revenge, but at the same time, he cannot condone it. He's been down the road many of those who summoned the Spear of Vengeance. He knows how it feels, but he would not sacrifice his soul just for revenge.
Truth relates to Kalista and respects her to a degree, but he'd probably have a hard time getting along with her.