Maharaja Atithi
Shri Ram and Mata Sita had many grandsons and Lava and Kusha had many children but the most famous grandson of Shri Ram is Atithi, son of Kusha and Naga Princess Kumudvati.
The imperial line of Raghu in Ayodhya was suffering from a crisis as Kusha’s first wife Champika (daughter of Maharaja Bhurikirti) had nine daughters but unable to have sons (once Rishi Agastya had prophesized the same to Shri Ram and predicted advent of Kumudvati as love of Kusha and mother of his sons).
Mahadevi Kumudvati
After Shri Ram left the mortal world, one day after a pleasure tour in Sarayu river with his wife Champika (and other Darakas), he lost a golden bracelet in the river. Servants thought it was taken by Nagas so he brought out Garudastra to punish them but Naga King Kumud returned it saying that his sister Kumudvati took it by mistake and wanted to marry him to atone her mistake. Kusha gladly took Kumudvati as his wife and the Devas blessed the union.
Kumudvati gave birth to eight sons and a daughter named Champakmalini. The eldest son was named Atithi who was very handsome and intelligent. He was apple of his father's eyes being his first son and also worthy of the throne. Kusha's love for Atithi was just like love of Dhritarashtra for Duryodhana minus all the toxicity.
When Kusha died, Kumudvati also decided to follow her husband in death as she couldn't live without him. Atithi lost both his parents on the same funeral pyre. He was crowned King and his Chief Queen was his favourite wife, a Nishadha Princess. Atithi had a son with her whom he named Nishad to honour her lineage and he became the next King after Atithi.
Source- Ananda Ramayana and Raghuvansh by Kalidas