Becky Thatcher is Tom's love interest. Tom is obsessed with her. Heck, he starts to "'show off' in all sorts of absurd and foolish ways," from the moment he sees her (1.12). But their relationship is really just one of those hot-and-cold schoolyard romances. One minute Becky is in love with Tom, the next she's crying. When she and Tom get lost in the cave, she does little more than pout and cry and collapse from fatigue.
She does occasionally show a bit of spunk, putting Tom down with a brilliant comeback or, in her shining moment, pretending to read a picture book with Alfred Temple, the snobbiest kid in school, in order to get back at Tom. But she never gets the chance to shine on her own. As such, she feels a bit too much like an object sometimes; she less an equal of Tom's than an idol for him to worship.