Hello! I created the retro achievement set for this game a little while ago and just found in the games code where the level up stats are located, so I thought I would share them here for anyone curious. While I thought the characters were balanced, turns out there is quite a difference. (This is at least in the north american release, haven't looked into the other two, but haven't noticed any other differences so I don't expect this would be either)
Bro [Max Fishing Tech/Walking Speed/Swimming Speed : 5/5/5]
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Fishing Technique upgrades at levels: 6, 22, 31, 38
Walking Speed upgrades at levels: 15, 25, 46, 60
Swimming Speed upgrades at levels: 8, 36, 52, 70
Father [Max Fishing Tech/Walking Speed/Swimming Speed : 5/4/5]
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Fishing Technique upgrades at levels: 19, 30, 52, 89
Walking Speed upgrades at levels: 14, 34, 77
Swimming Speed upgrades at levels: 8, 26, 48, 64
Mother [Max Fishing Tech/Walking Speed/Swimming Speed : 4/5/4]
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Fishing Technique upgrades at levels: 8, 16, 45
Walking Speed upgrades at levels: 5, 30, 78, 86
Swimming Speed upgrades at levels: 14, 24, 68
Sister [Max Fishing Tech/Walking Speed/Swimming Speed : 5/4/5]
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Fishing Technique upgrades at levels: 18, 35, 79, 94
Walking Speed upgrades at levels: 15, 19, 48
Swimming Speed upgrades at levels: 9, 28, 50, 65