Lord Vishnu, the preserver, is one Trimurti, beside Brahma (the creator) and Shiva (the destroyer). Vishnu ji is the preserver and the protector of the universe. He maintains the universe and restores balance whenever any evil forces try to destroy it. Lord Vishnu can be manifested in many forms, but he has ten popular avatars. Here is the Vishnu naam mantra:
Om Namo Narayanaya
ॐ नमो नारायणाय
Om Namo Narayanaya is a popular mantra in Hinduism, which is directly addressed to Vishnu ji. The mantra is changed to connect with Lord Vishnu and get his blessings. It also benefits in getting peace of mind and spiritual development. As it is a naam mantra, anyone can chant this mantra and take the blessings of Lord Vishnu.