Bud is a male merostomatozon and he is Lindsey's husband. He's a small crab-like creature who is said to be guarding the Year Nine boys' dorms. He is introduced in Faraway Morning. Unlike Lindsey, Bud doesn't seem to need any device to communicate, as seen in his introductory page he is able to talk directly to the kids with no trouble at all. Despite being held in a bucket, he is actually a very powerful psychic, being able to keep track of everyone out of his sight and communicating back and forth with Lindsey from a faraway location through the ether. Along with this he is knowledgeable in science, at least enough to hold his own in a conversation with Kat.
Bud looks like a red and blue crab, it's apparent that male merostomatozon are a lot smaller than their female counterparts. Bud's appearance in the ethereal is much larger than his physical form, spawning more legs, complex markings and a wispy tail. He still pales in comparison to Lindsey's ethereal form, however.
Bud and Lindsey are likely a reference to the movie The Abyss (Wikipedia).