Described by Grace as "the heavy younger sister", Joyce is the youngest of the Adler siblings.
Grace mentions that due to Joyce gaining weight, their mother made her wear ankle weights to vacuum the living room, causing her to lash out on Grace.
Though she is mentioned several times through out the series, Joyce first appears during the flashback episode Lows in the Mid-Eighties where she was still in high school and living with her parents in Schenectady. She is implied to be bitter and resentful of Grace partly because of her crush on Grace's then-boyfriend Will. Grace mentions that she binge eats when stressed, probably due to an eating disorder.
In their adulthood, Joyce has shed her weight, and is still attracted to Will.