The baby might be very inquisitive. With Athena as its mother, it would inherit a thirst for knowledge. It could constantly be asking questions, eager to learn about the world around it, whether it's the secrets of the ocean (from Percy's side) or the wisdom of the heavens.
Given the combination of Percy's impulsive nature and Athena's calculated wisdom, the baby could have a very interesting personality. Sometimes it might act on impulse, like jumping into a situation without thinking too much. But then, Athena's influence would kick in, and it would quickly analyze the situation and find a more intelligent way to deal with it.
The baby could inherit Soul's coolness. Maybe it would be a laid - back baby who is not easily flustered. Or it could have Maka's determination. So when it wants something, like a toy, it would be very persistent in getting it.
They might have a baby that has a combination of their physical features. For example, it could have Foxy's red fur and Mangle's long limbs. Maybe it also has Foxy's eye patch on one eye and Mangle's multiple tails.
Another possibility is that the baby has a combined power. It could use water to create illusions, a mix of Percy's water control and Annabeth's strategic thinking. So when in a fight, it could create false images of itself using water to confuse the enemy and then attack from an unexpected angle.
Maybe it has a more reserved personality like Branch at first. A bit shy but with a big heart. As it grows, it could start to come out of its shell more, especially with the help of its parents and the friendly Trolls around. It could be really creative, using things from the forest to make little artworks.
One possibility is that the baby could inherit unique powers. Since Percy has his water - related abilities and Athena is the goddess of wisdom, the baby might have the power to control water in a very intelligent way, like using it to solve complex puzzles or defend in a strategic manner.
Since Katara is a strong - willed and empathetic person, the baby could inherit her compassion. They would be the type to help others in need, perhaps using their bending abilities to do so. Their personality might also be a blend of both parents' determination, making them a force to be reckoned with when they grow up.
I think the baby could be quite resourceful. Finnick was good at using his charm and skills to survive, and Katniss was always finding ways to get through difficult situations. So, the baby would likely inherit that ability to make the most of what's available. It would also probably be very protective of those it loves, having seen the sacrifices its parents made for each other and their friends.
The baby might be really competitive, getting that from both of them. Vegeta is always striving to be the best, and Goku loves a good fight. But it could also be really kind - Goku's a good - hearted guy, so that would rub off. Maybe a bit of a goofball too, like Goku.
The baby could be really feisty. Since Sesshomaru is a powerful demon, the baby might inherit some of that strength and show it early on as a spunky and fearless personality.
It could be a very intelligent baby. Leela is smart and Fry has had his moments of unexpected wisdom (or at least knowledge from the future). So, their baby could be a quick learner, showing an early interest in things like science or technology. Maybe it starts building little gadgets at a very young age, much to the surprise and pride of its parents.