Henry "Ace" MacDoggin is an adult dog monster resembling a beagle. He has the typical beagle markings, tan and white with a black saddle-like mark on his back. He stands on all fours but can use his front paws like hands and stand on his hind legs for short amounts of time. He usually wears a tan trench coat and matching fedora or no clothes at all.
Despite being an adult in his late twenties, Ace is very immature. He tends to view everything as a game. He's also very naive and not very smart. Despite all of this, he somehow manages to do pretty well at his job as a private detective. Ace considers himself to be a renegade who plays by his own rules but he's also very loveable and kind towards others.
Ace was once a proud member of the Royal Guard's Canine Unit. He and the other dogs got along fantastically and the other members liked him as well. One night though, Ace noticed a tall skeleton carrying a bone and heading towards Undyne's house. The skeleton spotted him and was afraid the dog would attack him so he tossed the bone and hurried off. Ace caught the bone and ran after him to thank him but he was nowhere in sight. He started to go back towards Snowdin when he realized there was a good chance the other dogs would get jealous so he decided to bury it in the safest place possible, Undyne's yard. Once he was done, he quickly ran back to his house in Snowdin.
Unfortunately, the Mad Dummy was there posing as Undyne's training dummy. He told Undyne and she had no trouble uncovering the bone as there was a pile of dirt on top of it. She called Ace to her house and gave him a stern lecture. He denied it and grew angry. He finally decided to quit. He opened his own private detective agency where he could follow his own rules. To this day, he still holds a strong resentment for the Royal Guard.
- Not mentioned
- The other residents of Snowdin: Ace has helped many residents by solving mysteries such as who stole one of Grillbys mugs or who drew graffiti on The outside of the Inn.
- The Royal Guard: Ace resents the Royal Guard and believes them to be to stiff and orderly.
- Ace MacDoggin is based on Bert Macklin/Andy Dwyer from the TV show Parks and Recreation.
- The idea for Ace came aroundbetween late December and early January. His name was originally going to be Jack MacDoggin and he was going to be a husky.