Lord Vishnu

Lord Vishnu (Sanskrit pronunciation: [vɪʂɳu]; Sanskrit: विष्णु) is one of the principal deities of Hinduism, and the Supreme Being in its Vaishnavism.
Vishnu is reagrded as the "preserver" in the Hindu trinity (Trimurti) that includes Brahma and Shiva.
He is worshiped as the protector from evil, chaos and destruction.
Bhagwan Vishnu or Lord Vishnu is described to have numerous "avatars", though ten of his incarnations (Dashavatara), are celebrated therein as his major appearances.

The Bhagavad Gita (Chapter 4, Verse 7) describes his appearances-

यदा यदा हि धर्मस्य ग्लानिर्भवति भारत |

अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् ||

yadā yadā hi dharmasya glānir bhavati bhārata|

abhyutthānam adharmasya tadātmānaṁ sṛijāmyaham||

(Translation:- Whenever there is a decline in righteousness and an increase in unrighteousness, O Arjun, at that time I manifest myself on earth.)