Dr. Caitlin Snow is a bioengineering expert and a crucial part of Barry Allen's S.T.A.R. Labs support team.

During that fateful explosion at S.T.A.R. Labs she lost her fiancé, Ronnie Raymond. At least that is what she believes when she meets Barry Allen. The character is described as the ice to Ronnie's fire, foreshadowing their future metahuman abilities from the comics.

Dr. Caitlin Snow/Killer Frost Quotes

Caitlin: Ronnie's alive. I saw him Cisco. I think he was following me...
Cisco: Caitlin, Ronnie died. He was vaporized in the accelerator when it went critical.
Caitlin: Ronnie's alive and he's out there, alone and scared and I need your help to find him.

Caitlin: Since when do we have facial recognition software?
Felicity: Happy Hanakuah.

The Flash Quotes

One mystery I cannot figure out is why some people come into our lives and why some people go. Others become a part of you. Some friendships feel like they'll last forever and others end far too soon. Not every friendship is meant to last forever. What does last forever is the pain when that person is gone.

Barry

Barry: I love you, Iris.
Iris: Oh. I love you, too.
Barry: When we were kids I loved you before I even know what the word love meant and then my mom died and I had to go live with the girl I had a crush on. Look. There were so many times I wanted to tell you; Jr. Prom, when I went away to college, when I came back from college, nights that we stayed up talking, all the birthdays, all the Christmases, but I, I never did. I kept it in. After I lost my mom and my dad, I was afraid that if you didn't feel the same way, I would lose you, too. That's the irony. I was so scared of losing you that I did. I know I've had our whole lives to tell you this and you're with Eddie now and I know that and I know my timing couldn't be any worse, but I just... I couldn't lie to you anymore. I'm sorry.