Summer Lane is the oldest child of the Lane family.
Summer has four children, two of which are seen in the series, Courtney and Adrian. All her children take every opportunity to run away from her, and she's had at least one failed marriage. She has described the Lane Family home in Lawndale as the last place she wants to be.
Summer has shared the Lane's off-hands parenting methods. As a child, she was once allowed (i.e., Amanda did not care) to eat nothing but Pez for a whole year (Lane Miserables). As a teenager, she attended Lawndale High and was loathed as a slacker by Ms. Morris (See Jane Run).
Adult Summer does not live in Lawndale at the time of the series, though Jane mentions a younger Summer living at home with some of her children - Jane used to help manage them until "they became old enough to run away" (Pinch Sitter). She has four children (The Teachings of Don Jake), though only two of them, Adrian and Courtney, are shown in the series or the books.
Her children run away from home periodically and she has to get private detectives to find them.
Why they run away is unknown, though in The Daria Diaries she abruptly dropped her other two children at the family home (when nobody was in!) while tracking Adrian and Courtney. It is unknown if she's married still, but a comment by Penny refers to Summer having "experience" at failed marriages.
Summer shares the rest of her siblings' tense relations with one another. When the Lane family is together in years (Lane Miserables), Summer only appeared because Adrien and Courtney had run off to Casa Lane. Once there, she bickers with her siblings, showing contempt for Penny and berating her children for leading her there, stating the family home is [sic] "the last place on Earth I want to be".
In an interview with Kara Wild, Glenn Eichler has stated that Summer was the "Rita Barksdale" of the Lanes: someone who'd "[lost] their footing a bit, whether that meant dropping out of school, becoming a huge druggie, getting into a bunch of bad "all you need is love" relationships, or having any number of other experiences that their parents considered scandalous" (though without the "considered scandalous" part, knowing the Lanes).
As Summer is in her early-to-mid 30s, it could mean she was at school (maybe not the same year) at the same time as Kevin's mother and father.