This is probably not balanced.
HP as Magic-User, Experience Table as Elf, and Saves as Specialist. The April Fool must be Chaotic in alignment.
Starts with 2 in 6 in both Stealth and Sneak attack. Both increase to 3 in 6 at Level 4, 4 in 6 at Level 7, 5 in 6 at Level 10, and 6 in 6 at Level 13.
For the purposes of spell effects, the April Fool is treated as an Elf.
A number of times per day equal to the character's level, the April Fool can attempt to get away with one crime, prank, or social faux pas by saying the words "April Fools!" If it actually is April 1 at the April Fool's location, this attempt succeeds automatically. Otherwise, the April Fool must roll 3d6; if the April Fool rolls equal to or lower than their own Charisma score, all NPCs within earshot will forgive the April Fool's transgression and laugh heartily. They will also move one step more friendly on the Reaction chart. If the April Fool rolls higher, all NPCs within earshot will become one step more hostile on the Reaction chart.
NPCs who are subject to a successful "April Fools!" attempt do not need to be able to understand the words of the April Fool - they just need to be able to hear them. NPCs who are normally incapable of hearty laughter temporarily gain this ability for 1d10 minutes.
The "April Fools!" ability can only allow the April Fool to get away with something they did within the last hour.
A successful use of the "April Fools!" ability will essentially alter the minds of NPCs in two key ways: first, they will find the normally-offensive action of the April Fool extremely funny, and two, they will forgive the April Fool for that action. For example, if the April Fool murders a pastor in front of his entire congregation and shouts "April Fools!" immediately (and succeeds on the roll of 3d6), everyone in the congregation who can hear the April Fool will break into laughter and be perfectly okay with the murder. They will not attempt to harm/capture/press charges against the April Fool as a result of the murder, and they will otherwise hold no ill will against the April Fool for this action. When they tell other people about the event, they will recount it as if it were a hilarious joke. People in the congregation who are deaf or who ran out of earshot as soon as the murder occurred will, of course, continue to react normally.
A successful use of the "April Fools!" ability must be targeted at absolving a specific action committed by the April Fool. Lesser crimes directly connected to the forgiven action will also be forgiven, but major or unrelated crimes may not, at the discretion of the DM. For example, if the April Fool breaks into a house, smothers a sleeping man to death with his own toupee, goes downstairs to steal the rare antiques in the parlor, and is discovered by the dead man's wife while stuffing her grandfather clock into an over-sized bag with a dollar sign on it, the April Fool can use the "April Fools!" ability to make her forgive the theft (and the trespassing that was instrumental in that theft), but not the murder (or vice versa).
The April Fool always begins play with a stack of paper fish and a roll of tape.