Anwold is one of Cedric's servants, his torchbearer. When Ivanhoe refuses to share his gossip from the Holy Land with Cedric's servants, Anwold gets really huffy. He also makes several anti-Semitic remarks about Isaac. So all around, Anwold is kind of a jerk.
Balder is one of Cedric's favorite dogs, an elderly wolfhound. He likes to sit next to Cedric in his dining hall. In Norse mythology, Balder is the son of Odin, the leader of the gods. The fact that Cedric names his favorite dog after him is further proof of how much he values his Germanic, Saxon heritage. No ancient French or Norman names for him.
Elgitha is one of Rowena's ladies-in-waiting. She appears to be Rowena's most trusted lady, since when Rowena pulls "the Pilgrim" (Ivanhoe in disguise) into her rooms to ask for gossip about Ivanhoe, she's the only other person who gets to stay. Unfortunately for Elgitha, she is also with Rowena when she gets kidnapped, but she apparently survives the siege of Torquilstone. The next we hear of her is in Chapter 44, when she brings Rebecca to Rowena's rooms for a final chat.
Fangs is Gurth's large, ragged-looking dog. If his name sounds familiar, it's because Hagrid has a similarly large dog in the Harry Potter series, also called Fang. Ivanhoe's Fangs came first, of course, predating Harry Potter by about 180 years.
Fangs plays a very small part in the book. The most notable thing about him is that Cedric tries to spear him when Gurth blames his disappearance (when he goes to help Ivanhoe at the tournament) on Fangs running off. Gurth swears never to forgive Cedric for trying to kill his dog, but as soon as Cedric is kidnapped by the Normans, all is forgotten. Still, we think pretty poorly of Cedric for this – it's a low blow, throwing a spear at a man's dog.
There is a historical Hereward, called Hereward the Wake, who was an 11th century Anglo-Saxon leader of the resistance against the Norman conquerors. Hereward is also the name of Cedric's father. The dates roughly line up, so Scott might mean them to be one and the same, but we can't say for sure.
One of Cedric's many servants. Hugo appears to be a guard, since he carries a lance (a long spear) around the castle.
Scott's lack of attention to detail strikes again: this character appears as Hundebert in Chapter 3, when he's the majordomo (basically a butler) at Rotherwood. But then Hundibert gets captured along with Cedric and Oswald as they're traveling through the forest. We assume Hundebert the Butler and Hundibert the unfortunate collateral damage are one and the same person. Clearly this character is not a central figure in Ivanhoe either way.
Oswald is Cedric's cupbearer. This literally means that he carries Cedric's cup, but it also means that he's in charge of keeping his lord supplied with wine.
Cedric, who enjoys his wine, clearly values Oswald as a servant and seems close to him. When Ivanhoe passes out at the tournament at Ashby, Cedric sends Oswald to find out what's happened to him. (He doesn't, since Rebecca is quick to sweep Ivanhoe up and carry him away to her house.) Oswald is also with Cedric when he gets captured in the forest by De Bracy.