In FR/LG, Professor Oaks aides can be found in the first floor of the the gate on Route 2 (requires 10 Pokemon), the first floor of the Pokemon Center on Route 10 (requires 20 Pokemon), the second floor of the gate on Route 11 (requires 30 Pokemon), the second floor of the gate on Route 16 (requires 40 Pokemon), and the second floor of the gate on Route 15. (requires 50 Pokemon)
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