The only qualification of a devotee is to be unflinchingly and exclusively engaged in devotional service.
In the Nṛsiṁha Purāṇa the following statement is given:

bhagavati ca harāv ananya-cetā
bhṛśa-malino 'pi virājate manuṣyaḥ
na hi śaśa-kaluṣa-cchabiḥ kadācit
timira-parābhavatām upaiti candraḥ
The meaning is that even if one fully engaged in the devotional service of the Lord is sometimes found engaged in abominable activities, these activities should be considered to be like the spots that resemble the mark of a rabbit on the moon. Such spots do not become an impediment to the diffusion of moonlight. Similarly, the accidental falldown of a devotee from the path of saintly character does not make him abominable.

(Bhagavad-Gita-------9:30------purport).

One who is not in Kṛṣṇa consciousness usually cannot sit silently even for half an hour. The yoga system teaches that if you become silent you will realize that you are God. This system may be all right for materialistic persons, but how long will they be able to keep themselves silent? Artificially, they may sit down for so-called meditation, but immediately after their yogic performance they will engage themselves again in such activities as illicit sex life, gambling, meat-eating and many other nonsensical things. But a Kṛṣṇa conscious person gradually elevates himself without endeavoring for this so-called silent meditation. Simply because he is engaged in Kṛṣṇa consciousness he automatically gives up all this nonsense and develops a high character. One develops the highest character by becoming a pure devotee of Kṛṣṇa. The conclusion is that no one can truly have any good qualities if he is lacking Kṛṣṇa consciousness.

(Nectar of Devotion).

One who approaches the Supreme Lord to render devotional service, but who is proud of his personality, envious of others or vengeful, is in the mode of anger. He thinks that he is the best devotee. Devotional service executed in this way is not pure; it is mixed and is of the lowest grade, tāmasaḥ. Śrīla Viśvanātha Cakravartī Ṭhākura advises that a Vaiṣṇava who is not of good character should be avoided. A Vaiṣṇava is one who has taken the Supreme Personality of Godhead as the ultimate goal of life, but if one is not pure and still has motives, then he is not a Vaiṣṇava of the first order of good character. One may offer his respects to such a Vaiṣṇava because he has accepted the Supreme Lord as the ultimate goal of life, but one should not keep company with a Vaiṣṇava who is in the mode of ignorance.

(Srimad Bhagavatam-------3:29:8-------purport).

The word caitanya means "living force," carita means "character," and amṛta means "immortal." As living entities we can move, but a table cannot because it does not possess living force. Movement and activity may be considered signs or symptoms of the living force. Indeed, it may be said that there can be no activity without the living force. Although the living force is present in the material condition, this condition is not amṛta, immortal. The words Caitanya-caritāmṛta, then, may be translated as "the character of the living force in immortality."

(Sri Caitanya Caritamrta--------introduction).

A brahmacārī has to live in the home of the spiritual master just like a servant, and he must beg alms from door to door and bring them to the spiritual master. He takes food only under the master's order, and if the master neglects to call the student for food that day, the student fasts. These are some of the Vedic principles for observing brahmacarya. After the student studies the Vedas under the master for some time—at least from from age five to twenty—he may become a man of perfect character. Study of the Vedas is not meant for the recreation of armchair speculators, but for the formation of character. After this training, the brahmacārī is allowed to enter into household life and marry. When he is a householder, he has to perform many sacrifices so that he may achieve further enlightenment.

(Bhagavad-Gita-------8:28------purport).

Mahārāja Yudhiṣṭhira, as a good soul and devotee of the Lord, was always conscious of the great loss of his aunt and her sufferings as an ascetic. An ascetic is never disturbed by all kinds of sufferings, and that makes him strong and determined on the path of spiritual progress. Queen Gāndhārī is a typical example of an ascetic because of her marvelous character in many trying situations. She was an ideal woman as mother, wife and ascetic, and in the history of the world such character in a woman is rarely found.

(Srimad Bhagavatam-------1:13:39-------purport).