He was revealed to be a weak little toddler at the start of Z.
But he's apparently real strong when he's pissed.
So after Piccolo kills his father and uncle, he helps Gohan train.
Apparently not hard enough.
He is more of a punching bag for awhile.
He starts to shine in the Androids saga by being slightly useful.
Then he turns Super Saiyan.
Now he is apparently stronger than Goku.
Then he is stronger than anyone.
Easily kills the Cell Jr. gang.
And defeats the big daddy himself.
But after some bullshit, he does have to give it his all to beat Cell.
Later he becomes an awkward teenager.
And he seems he's gotten a bit weak.
But I really liked the Mystic transformation. Until he gets absorbed.