An advocate of egoism. / An egocentric or self-centered person. / (nonstandard, by confusion of the similar words) An egotist (person who talks excessively about themself).

Egoistical.

plural of egoist

in an egoistic manner

Caring about oneself and the gratification of one's own desires rather than others. / (nonstandard, by confusion of the similar words) Egotistical (tending to talk excessively about oneself).

an advocate of rational egoism

plural of rational egoist