[C_r_e_a_m_p_u_f_f]

I have a strong feeling it is given to the Emerald trainer for completing the Hoenn Dex. And in the Kanto and Hoenn regions, what happens to the last starter? The Hoenn one is given to Steven and he gives it to you for choosing a stone. Of course, Oak keeps the last one and refills the other two so that he can make the choice for you.