A ṛṣi (rishi; sage). In Vanaparva of Mahābhārata, it is said that the Pāṇḍava, while travelling across several sites of pilgrimage, reached Gandhamādana Parvata. There they found the hermitage of Maharṣi Ārṣṭisena.

When they reached there, Āṛṣṭiṣena welcomed all of them. Pāṇḍava(s) lived for some time in the hermitage of Āṛṣṭiṣena.
The Sage Ārṣṭiṣena lived in the mountain Gandhamādana. The beautiful Alakāpurī of Kuvera can be seen from this place–
atikramya ca taṃ pārtha ārṣṭiṣeṇāśrame vasa.
tato drakṣyasi kaunteya niveśaṃ dhanadasya tam.

Ārṣṭiṣeṇa was a ugratapasvī, his body was withered due to high ascetic practices–
tataste tigmatapasaṃ kṛśaṃ dhamanisantatam.

Along with the priest Dhaumya, and other Brāhmaṇa(s), when the Pāṇḍava(s) reached his hermitage, Ārṣṭiṣeṇa welcomed him with affection and cordiality. Ārṣṭiṣeṇa told Yudhiṣṭhira, Arjuna would come to the Gandhamādana mountain from heaven. As long as Arjuna would not come back, the Pāṇḍava(s) could comfortably stay at his hermitage.
vasadhvaṃ pāṇḍavaśreṣṭhā yāvadarjunadarśanam.

However, in the long description of the Pāṇḍava(s)̍ stay in the sage̍s hermitage, we don̍t get his detailed identity anywhere. Only in one śloka, he has been referred to as a Rājarṣi, from which it can be considered that Ārṣṭiṣeṇa was born in some Kṣatriya family, and later, by virtue of ascetic practices, he attained the status of a Brāhmaṇa.

The legend of Ārṣṭiṣeṇa̍s attainment of the status of a Brāhmaṇa, has been described in Śalyaparva of Mahābhārata. Travelling acorss the sites of pilgrimage, Valarāma one day reached Kapālamocana tīrtha. Describing the glory of this site, the poet of Mahābhārata writes, Ārṣṭiṣeṇa attained the status of a Brāhmaṇa, by practicing tapasyā in this very site–
yatrārṣṭiṣeṇa kauravya brāhmaṇyaṃ śaṃśitavrataḥ
tapasā mahatā rājan prāptavānṛṣisattama.

The Mahābhārata-poet has described– Āṛṣṭiṣeṇa, born of a Kṣatriya clan, lived in the house of the guru for many a days,and tried to be well-versed in Veda and Vedānta. However, despite his hard work and diligence, he was not able to achieve success. Much agrieved, he left the hermitage of the guru, went out to practice tapasyā at Kapālamocana tīrtha, on the bank of the river Sarasvatī, and as a result of that tapasyā, he attained the knowledge of all Veda and Vedānta, and became a brāhmaṇa. Ārṣṭiṣeṇa became so happy after attaining ̍siddhi,that he granted three boons to this place. He said, from that day onwards, if one takes a bath at this tīrtha on the bank of river Sarasvatī, one will achieve the fruit of Aśvamedha yajña. In this site, there would be no fear of any wild animal, and there, even practising a small amount of ascetic penance, one would be able to achieve siddhi. In Śalyaparva, however, it has been said that, after this, Ārṣṭiṣena left his mortal body and attained heaven.

In several places and incidents in Mahābhārata, the presence of Ārṣṭiṣeṇa can be found. When Pramadvarā, daughter of sage Sthulakeśa died of snakebite, Ārṣṭiṣeṇa was one who was present at the site, along with other sages.

In Śāntiparva of Mahābhārata, the name of Ārṣṭiṣeṇa has been remembered with great respect as a sage, a knower of Brahman. In Anuśāśanaparva, the hermitage of sage Ārṣṭiṣeṇa has been referred to as a sacred s