One and Two makes Four
It's all about Fleur, isn't it?
Fleur the flower attracts men as if they were bees. No, that doesn't sound right. Fleur the princess has knights swooning at her feet. Nor does that. Fleur the silkensmoothiceblondqueenofhearts - well. What does it matter, anyway? Tonks has never had a way with words, but the point is that Fleur is what Tonks is not.
Bill is cool, not cold. Dark red hair and flashing grin, cursebreaker, ladies man. He is charismatic, and good in a fight, but he hates conflict. When things get tough, he moves on to something easier. He and Fleur look good together, like a couple out of a fairy tale - the beautiful princess and the dashing prince. They are often seen together during the day, ostentatiously together, as Bill loops an arm around Fleur's slender waist, but when they smile at each other, their eyes don't seem to meet. Tonks pretends not to notice. After all, they make a pretty picture.
Remus is the epitome of an English man - quiet, dignified, reserved. Wise, perhaps. Tonks isn't sure what wisdom looks like these days. He is much older than Fleur, and not very handsome. It doesn't matter, as they only ever meet at night. In the dark, Fleur doesn't look like royalty anymore. When she climbs the stairs of Grimmauld Place after midnight she looks young and vulnerable. She slips into Remus' room and finds someone to take care of her, or so Tonks assumes. Tonks thinks she can understand that. The only other answer she can come up with is that Fleur just wants to monopolise all the available men.
Tonks is clumsy and boyish. She can change her face, and she can change her body, but no matter how beautiful she looks she will still make a fool of herself by falling over the umbrella stand. In the day she doesn't mind so much. She is loud and boisterous and popular with the kids. She wears her hair bright pink whenever she can. And if her teeth clench involuntarily when Bill and Fleur fill the room with their presence, or when she sees Remus studiously not looking at Fleur, well, it's not like anyone is looking at her closely enough to notice.
Every night, Tonks turns her hair and skin grey and hides to watch Fleur creep up the stairs, and sometimes, when the night is especially dark and she can see the warm light under the crack of Remus' door, she wonders whether it's Fleur or Remus she is jealous of.