Final Fantasy VII: Yuffie Kisaragi
Although Yuffie was an optional character in Final Fantasy VII, she made appearances in several other titles and spin-offs of the original game, such as Advent Children, Dirge of Cerberus and the Kingdom Hearts series, among others. Yuffie starts off as a sneaky, arrogant little materia thief with no apparent noble reasons to support her behavior; she thinks herself to be completely self-sufficient and seems determined to avoid creating bonds with anyone: in this sense, she is nothing like our Selphie, who, despite being independent, retains an innate warmth towards other people. However, she slowly begins to change, eventually realizing there is no denying it: friends are a good thing to have around, and it's okay to depend on others (in fact, by the time Advent Children starts, she seems to have had no trouble making up for the wasted time as far as fitting in goes).
Either way, what truly approaches Selphie and Yuffie as characters is the active, energetic aura about them; they are lively, spunky and often act immature, working as the comic relief in their respective games. Maybe even more importantly, they are both extremely fond of their roots (Wutai in Yuffie's case, Trabia in Selphie's), and will do anything to help protect the place they grew up in: Selphie attempts to stop the missile launch against Trabia Garden and, soon after her inevitable failure, goes to visit the ruins, determined to provide all support needed; likewise, as the player comes to know, Yuffie's sole reason to act the, well, unpleasant way she did was her strong wish to restore Wutai to its former glory
Physically, these two don't look alike in the slightest, apart from the constant smile/smirk (the latter coming up a lot more in Yuffie's case). Nonetheless, Yuffie is undoubtly the first in a widely recognized pattern of hyper female teenage characters in Final Fantasy games, probably having served as a basis for Selphie in many ways.
Final Fantasy X/X-2: Rikku
To put it simply, Rikku represents serious competition to Selphie in what concerns being the happy-go-luckiest female character in the whole Final Fantasy series. Similarities don't ever seem to end: both are spunky, lively, dedicated, determined, sweet and optimistic; they possess a great sense of patriotism for their respective fatherlands (the Al-Bhed Home in Rikku's case and Trabia in Selphie's), as well as a strong will and team spirit. As if this were not enough, both girls have a flair for electronics/machines, a slight tendency to say cute things such as "aww, bummer!", and, on a more physical note, vivid green eyes (although, other than that, they don't look alike).
If I had to explain Rikku's personality in only a few words (not that I seriously find that possible to do, mind you: Rikku is a favorite!), I'd say she is like the result of a successful fusion between her two predecessors in the "hyperactive female" pattern, Yuffie and Selphie: in short, she is a spunky thief who does not rob her own party and believes happy is good. Rikku and Selphie are both very selfless, in the sense that they are willing to go to extremes to defend the people they care for and believe in, regardless of consequences. They are sociable, sophisticated and playful, and even share having started their own lingoes ("disasterrific" for Rikku, "booyaka" for Selphie). In fact, the resemblance is so strong that, when faced with the task to find a substitute character for Rikku in the original Kingdom Hearts game (more here), Squaresoft went for Selphie.
They sure know how to sweep a girl off her feet!
Bishoujo Senshi Sailor Moon: Aino Minako/Sailor Venus
Sailor Moon was my very first fandom. Back then, Minako (Sailor V, Sailor Venus) wasn't a top favorite, but she eventually grew on me. Looking back on her now, and, of course, analyzing beyond the innocence that is common to all characters in this series, I can't help but notice the huge amount of personality traits she shares with Selphie.
Long blonde hair and clear blue eyes aside, Minako resembles Selphie in the way she is cheerful, involved and happy: "if you fall, lift yourself up" is her motto, and this greatly suits Selphie's optimistic outlook on life. Furthermore, being the senshi of love, Venus, like Selphie, possesses an innate ability to connect, socialize and reach out to people; she can also get a little wild and out of control at times, often adopting a hyperactive attitude.
Because Minako is slightly clumsy and more often than not seen chasing after random hot guys, she is constantly mistaken for a dumb blonde with no real depth: on the contrary, she takes her mission as a senshi very seriously, and is a warm, sensitive person who will do whatever it takes for those closest to her to be happy. Even when she is down, Minako takes whatever energy she has left and either spreads it around or turns it into something productive. All of this is very reminiscent of Selphie; in particular, many people fail to see the strong, balanced woman beyond the klutzy teenage girl who drools over Laguna, thus missing out on Selphie's true, rather admirable core.
On an interesting side note, Minako's blood type is (accurately) B, like Selphie's, and both characters are strongly associated with bright, lively colors, especially yellow.