Who is the Japanese voice actor for Isamu?

The Japanese voice actor for Isamu is Makoto Tsumura.

Which anime is Isamu from?

Isamu appears in Pokemon Diamond & Pearl.

In Pokemon Diamond & Pearl, Isamu is listed as a supporting character.

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Ryan and Bryan are twin brothers and the characters of the day who appeared in The Champ Twins!. (Source: Bulbapedia)

Pokemon Diamond & Pearl

Makoto Tsumura

Japanese

Emlyn Morinelli

English

Julie Basecqz

French

Massimo Di Benedetto

Italian

Raphael Ferreira

Portuguese (BR)

Víctor Ugarte

Spanish