Simon Ian Rhodes II (voiced by Alan Marriot) is the older brother of Stanford, and 188th in line for the British throne. Outwardly, he seems quite self-absorbed, even worse than Stanford was at the beginning of the series, and doesn't seem to have much of a personality beyond his overblown ego.
As pointed out by Agura in StormShocker, the Rhodes brothers look identical to each other (although Simon brags he's better looking than Stanford), the only real physical difference between them being that Simon's eyes have flecks of gold in them. Two other ways to tell the brothers apart is that they style their hair differently and they also dress differently as well.
He and his brother have an extremely competitive relationship. But despite Simon's unlikeable nature, Stanford still cares for him dearly.
It's never stated how far apart in age Simon and Stanford are. Simon doesn't seem to be that much older than Stanford, so the brothers are probably only a year or two apart in age.
He seems to have developed a crush on Grace in StormShocker, as he flirts with her a lot in that episode.
In Full Throttle, it's revealed that Simon is friends with the professional racecar driver.