Despite it being a foregone conclusion, the way Lariatte becomes Yvienne's familiar is really touching. Yvienne opened her field accidentally, and with no familiar had resigned herself to dying. Lariatte refused to let her, and decided if Yvienne couldn't think of a reason to want to live, then she'd do it for her.
Lariatte: Listen, Yvienne. Is a life that wants nothing such a bad life? Is it bad to live a vain life? Making a decent living after graduation. Just working hard like other people do and going to a park on weekends. Getting married to a decent guy, and living in a house that's not too big or too small. And so many things will occur. There will be happy days and also sad days. [...] S... Sorry, I sound so stupid? I don't even know what I'm trying to say...
Yvienne: Lariatte, thanks. For the first time I was able to see my own future. I was able to see it!
After doing his best to avoid him, Daughter wakes January from a nightmare and by comforting him in the aftermath the two manage to come to a closer understanding. The next morning, he finally decides to stop letting January's insane family be an issue and becomes his friend.
Over the summer break, Daughter decides to surprise January with a visit and crash his family reunion. January's first impulse? To run all the way through his mansion just to full-out throw himself at him.
January: Nice to meet you again. It's been nine years.
When Newton is about to be destroyed by the enormous March Hare and Yvienne's powers alone aren't enough to defeat it, Lariatte chooses to risk her life to defeat it and go down fighting alongside her.
And again in volume 12, Lariatte shows the true extent of her devotion to Yvienne by ultimately being the one to find her when Yvienne's trapped inside her own mind, and literally bringing light to the endless abyss that is Yvienne's soul.
Volume 14 is pretty much a series of terrible things happening to Daughter and January, triggered by Daughter successfully getting revenge and being arrested for it. But January doesn't give up on trying to save him, even to the point of selling himself as a mage to the king. When Daughter comes to save him, the look they give each othersays it all.
And later, when they're both out of danger...
Daughter: January. I truly have nothing left for me to do. If you become a monster, I'll kill you. I'll keep my eyes on you until your last moments, so... you, and me... let's live. Let's live together.