who is the 23rd thirthankara

The 23rd Tirthankara in Jainism is Bhagwan Parshvanath (Parshvanatha / Parshwanath).

Very briefly about him:

• He lived before Bhagwan Mahavir (24th Tirthankara).
• Born in Varanasi (Kashi) to King Ashvasen and Queen Vamadevi.
• Known especially for strong teachings of ahimsa (non‑violence) and other vows.
• His lanchhan (symbol) is the snake (serpent).
• He is one of the most widely worshipped Tirthankaras; many Jain temples are dedicated to him.

You can read more about Bhagwan Parshvanath here