Older people with dementia have many difficulties while using Serious Game for their cognitive training. Besides of the lack of game culture and a limited acceptance toward new technology, memory and cognitive disorders influence negatively also on their performance. We propose a virtual agent that can enhance performance of player using engaging strategies. We assess the efficiency of the agent compared with a therapist through an experiment with 47 older adults distributed in two groups (Mild Cognitive Impairment and Alzheimer). The experimental results report that the agent can help participants producing similar performances (score, precision, learning effect) than in the case with therapist. Moreover, participants have appreciated the agent and desired to continue playing temporarily with the agent instead of the therapist.