Like ranma Dipper can turn into a girl with cold water and a boy with hot. Both are 16.
Gravity falls pool
Bill is seen talking with a bunch of girls till he gets splash by some pool water making the girls giggle and leave.
"Hey who did that!" Bill said getting angry and went to see who splashed him but stopped when he saw the one who splashed him... A beautiful brunette girl with an hourglass figure in a pink bikina staring at him with big delicious brown eyes and lips so pink he wants to kiss them.
"Sorry there I didn't mean to do that." The girl apologized smiling whole heartily making Bill's heart skip a beat unable to speak so the girl giggled swimming away leaving a blushing Bill.
"Uh its okay." Bill muttered stunned at what happened.
Boys showers room
It was late so Bill was the only boy in the pool bathroom that's what he thought till he saw steam coming from the back.
"Oh sorry man I didn't mean to interrupt...." Bill couldn't finish his eye widen seeing a cute boy standing there in his glory under a shower head washing his soft curly brunette hair and his body with the right amount of muscles.
"Huh?" The boy said opening his eyes which were brown like the girl's he saw earlier with the same beautiful lips "Oh hey man if you need to shower don't let me bother you." He said smiling kindly at Bill.
"Uh that's alright I'll shower later." Bill said face blushing again he speed walking out of there embarrassed.
Cipher residence
"Man two crushes in one day who am doing?" Bill said to himself looking out his window and out of nowhere saw the boy from earlier sitting on a bench down the street wearing a white shirt, white shorts an sandals "I'll show that kid who's boss." He said grinning running to the bathroom and filled a bucket with cold water.
"What are you doing?" Will his little brother asked sitting in the tub.
"Doing a prank." Bill said simply walking out the bathroom out the house and quietly approached the boy from behind and dumped the water on him making jump screaming "Ha I totally got you dude!" He laughed then stopped "Uhhh...." He said dumbly.
"Oh great look what you've down you jerk!" The boy... Well apparently girl from the pool earlier said clutching her white drenched clothes and ran away from the stunned Bill.
"What just happened?" Bill asked himself then noticed an ID on the ground and picked it up "Dipper Pines." He read amazed smiling softly he went back inside he had a name.