Krishna, like Rama, is a mythological figure. But behind every mythology there is usually some history. However, we need not go into the question whether Krishna was a historical figure or not.
As portrayed in our literature (in Sanskrit and in many Indian languages), Krishna is a multi faceted personality: a child god, a prankster, a thief (“makhan chor”), a lover, a philosopher and adviser to Arjuna (in the Bhagavad Gita), a military strategist, a diplomat, a universal being, etc.
In the Mahabharat we come across him as an adult, but we learn nothing of his childhood in that epic. That we learn from other works like the Bhagavad Puran (particularly in the 10th chapter), the Harivansha, etc. In these texts, his pranks, escapades, frolics etc. from childhood are mentioned, as well as his killing of demons like Pootna and Kaali Nag.
When the first nuclear explosion in the world took place in Alamogordo in New Mexico, USA, on the 16th of July 1945, the great nuclear scientist Dr Robert Oppenheimer, the head of the Manhattan Project (who was also a great Sanskrit scholar), on seeing the mighty explosion, instantly recited a shloka of Lord Krishna to Arjuna in the Bhagavad Gita:
“Now I am become death, the destroyer of the world”
https://www.youtube.com/watch?v=dus_M4sn0_I
Krishna's advice to Arjun in the Gita that one should do one's duty without seeking any benefits is well known.
Krishna is also described as a 'Ranchhod' i.e. one who abandons the battlefield - which he did when the more powerful Jarasandh approached Krishna's capital Mathura, and Krishna retreated to Dwarka. This is a technique used in warfare, particularly guerilla warfare: retreating before a more powerful enemy, and far from being cowardice, it is an act of military strategy.
The love of Krishna for the gopis of Vrindavan, Radha and others has been mentioned in detail in Indian literature e.g. in Jayadev's Geet Govind, which depict Krishna's 'Raas Leela', and these are shown in skits and plays in many places in the Mathura region during the Holi week (see my article).
https://www.youtube.com/watch?v=6l7-3HBnwiE
Several poets of the Bhaktikaal period in Indian literature are full of devotion to Krishna e.g. Hindi poets Surdas, Raskhan (who, though a Muslim, was an ardent devotee), Mirabai, etc.; Tamil poet Andal (whose work Tiruppavai is recited widely in Tamilnadu every year in the month of Maagh or January); Marathi poets Eknath and Tukaram, etc. In Bengal the Krishna bhakti cult was spread by Chaitanya Mahaprabhu.
There are many famous Krishna temples e.g. Dwarkadhish in Mathura, Banke Bihari in neighbouring Vrindavan, Nathdwara on the border of Rajasthan and Gujarat, Parthasarathy in Chennai, Guruvayoor in Kerala, etc. We can also include the famous Tirupati temple in these, as it is a temple to Vishnu, and Krishna is regarded as his avatar.
There is also a Krishna temple in Lahore, Pakistan, which my journalist friend Sajjad Azhar Peerzada often visits at the invitation of the priest Pt. Kashiramji.
Jai Shri Krishna!