We had to wait around till the sixth book for him, but Stephen finally shows up to spice up the latter chapters. Stephen’s physical attraction to Maggie, and the sexual tension between them, is pretty intense. And Stephen also brings the romance:
"They’re going to waltz again," said Stephen, bending to speak to her, with that glance and tone of subdued tenderness which young dreams create to themselves in the summer woods when low cooing voices fill the air. Such glances and tones bring the breath of poetry with them into a room that is half-stifling with glaring gas and hard flirtation. (6.10.5)
Stephen and Maggie are always aware of one another physically and they frequently find ways to be in close physical proximity to one another. Eliot writes that "Each was oppressively conscious of the other’s presence, even to the finger-ends" (6.6.6). Man, that is pretty intense. Stephen basically seduces Maggie – she frequently experiences the sensation of being "swept" away by Stephen, whether he’s singing to her or rowing her down a river.
So, with all the sexual overtones here, you’d think Stephen would be wildly interesting. The problem is that he’s kind of a flat character. He’s often a bit ridiculous when it comes right down to it. We meet him flirting over a pair of scissors with Lucy, for crying out loud. He sings songs to ladies like he’s in a karaoke bar. He’s always begging Maggie to stay with him since he’ll "die" without her. And, by the end of the novel, he’s writing really accusatory letters to his "one true love." Aside from his burning passion for Maggie, we never really learn that much about Stephen. He seems to be witty, and he likes having fun, but other than that he’s a bit of a blank.
And this is entirely the point. Stephen basically represents the sort of romantic dream world that Maggie longs for and finds so seductive. The quote about the waltz emphasizes this point: Eliot writes of "young dreams" and "poetry" and the types of images and ideas you would find in romantic fiction. And Maggie herself is largely approaching her relationship with Stephen as a work of fiction:
It was not that she thought distinctly of Mr. Stephen Guest or dwelt on the indications that he looked at her with admiration; it was rather that she felt the half-remote presence of a world of love and beauty and delight, made up of vague, mingled images from all the poetry and romance she had ever read, or had ever woven in her dreamy reveries. (6.3.4)
The above quote is from rather early in Maggie’s acquaintance with Stephen; but, even as she gets to know him better, Maggie still retains this romanticized view of him. Stephen was like a fictional character come to life for Maggie. She finds him, their relationship, and Lucy’s world of ease culture and comfort very... comforting. It’s like a nice little fairy tale, a break from reality.
But Stephen is also a real person with immature emotional responses and the ability to tempt Maggie sexually. When Maggie "wakes up" from the dream-like state Stephen places her in, she is agitated and even horrified at how she behaved with Stephen.
In the end, Stephen is an odd mix of bland romantic interest and dangerous seducer. He represents both an imaginative fantasy world that Maggie longs for, and the real world of sexuality that Maggie finds more alarming.