Kari was born and raised in Coventry, West Midlands. She attended Wyken Croft and Earlsdon Primary Schools and later went to Westwood School and Community College. She started dance lessons at the age of four in tap, modern dance, and gymnastics. She also took up ballet, musical theatre, and street dance, and continued this training until the age of 20. Before gaining a place at Drama Centre London, Kari also attended a local drama group in Coventry called St Finbarr's Youth Arts for a few years, which specialised in performing Irish plays.
Kari's experience in St Finbarr's Youth Arts led to her being part of the community cast in The Belgrade Theatre's production of The Wedding. Although Kari enjoyed acting, dancing became her main priority. It wasn't until she was doing her A-levels in French, Biology, and Drama & Theatre Studies that she realised that she wanted to pursue a career in acting. After leaving school, she moved to London to begin her course at drama school. She graduated in 2011 with a BA(Hons) in acting.
During the second term of her final year at Drama Centre London, Kari won the role of Lucy in The Inbetweeners Movie. She has since had various roles in a number of plays. She also starred in the fourth series of the BBC Three supernatural drama series Being Human and the second series of Silk. She formerly starred in the BBC Three sitcom Cuckoo, which premiered on 25 September 2012, as well as in The Musketeers, in which she portrays Constance Bonacieux. In 2013, she played Danielle in the new ITV comedy The Job Lot. In 2014, she reprised the role of Lucy in The Inbetweeners 2.