When Susie sang at Didi's baby shower in the first movie, it was the first time that she displayed her talent for singing, a talent that would later become the most important part of her character.
But in 2003, when the spin-off All Grown Up! was put into production, a press release for the show announced that "smart, spunky Susie (Cree Summer), still a crusader for what's right, and upstages Angelica as a singer with real talent", which implied an increased role for Susie in the new spin-off. Besides her singing talent and despite being tomboyishly beautiful, it was revealed that she also knows ten languages, as shown in the episode "Runaround Susie".
The very first episode of the regular run, "Susie Sings The Blues" (first US tx: November 29, 2003), concerned Susie and her singing talent. Since then, she has appeared in a majority of the episodes (although still in less than the other characters). This may be explained by the fact that by the time All Grown Up! premiered, Summer had less regular roles on other shows. Also, while Susie and Angelica still treat each other as rivals (especially at singing, see "Susie Sings The Blues" and "It's Karma, Dude!" for cases in point), the two actually hang out with each other. Angelica has become jealous of Susie's singing and tomboyish beauty. On the other token, Susie has become somewhat of a "best" friend to Angelica. Susie helps Angelica find her doll in "Saving Cynthia".
Angelica is Susie's best friend. Susie loves Angelica as her best friend but almost always looks for ways to compete with her, and while they are still best friends Susie always one-ups Angelica and sometimes makes bets on her failures and makes fun of her behind her back to the rest of the gang. But it never phases Angelica because she believes it's because she's jealous. In 'Susie Goes Bad Lite' Susie gives into peer pressure and has to prove how 'bad' she was. Kimi and Lil had to go get Angelica so she could save Susie because she almost fell off a telephone company building and Angelica had to climb up there to talk some sense into Susie so that she wouldn't get hurt and Susie wouldn't listen and they fell, and if Lil and Kimi hadn't ran after a police patrol car then Angelica and Susie would've been seriously injured or possibly dead.
Lil and Susie go to the mall and shop with Angelica and Kimi and hang out all the time. The two have always been close, and as a young child Susie used to protect Lil and the others from Angelica.
Susie and Kimi are very close friends. They always talk, have each others backs, like the same stuff, join the same clubs (unless it's cultural), and attend each others events.
Susie and Phil are close friends despite the age difference and the lack of interactions. Some fans even ship the two.
Dil and Susie don't appear to have a lot of interactions in the series but they are still good friends and support each other and go to each others events, and they know each other pretty well.
Tommy and Susie don't interact much, however they are in good terms and are still friends. They have been known to help each other and protect each other (usually from Angelica). They were a lot closer when they were younger but they are still very good friends and support each other in their ideas and adventures.
Chuckie and Susie aren't super close but are still good friends. Susie used to protect Chuckie from Angelica and for that her was grateful and when her Reptar cereal bars tasted horrible he didn't want to tell her because he wanted to protect her from the harsh truth.