During her childhood she was a romanticized Katherine, dressed by her misty-eyed, fussy mother in dresses that looked like ruffled pillowcases. By high school she'd shed the frills and emerged as a bouncy, round-faced Kathy - - - At university she was Kath, blunt and no-bullshit in her Take-Back-the-Night jeans and checked shirt - - - When she ran away to England, she sliced herself down to Kat. It was economical, street-feline, and pointed as a nail.
Source: wiktionary