Can one definitively assert that one of the Companions will enter Paradise without an explicit text to that effect?

A Companion is one who met the Prophet, peace and blessings be upon him, believing in him, and died upon that belief. These are generally promised Paradise, based on the Almighty's saying: ﴿And the first forerunners [in the faith] among the Muhajireen and the Ansar and those who followed them with good conduct - Allah is pleased with them and they are pleased with Him, and He has prepared for them Gardens beneath which rivers flow, wherein they will abide forever. That is the great attainment.﴾ [At-Tawbah:100], and His saying: ﴿Not equal among you are those who spent before the conquest and fought. Those are greater in degree than those who spent afterwards and fought. But to each Allah has promised the best [reward].﴾ [Al-Hadid:10].

As for those whose admission to Paradise is explicitly confirmed by a religious text, they are attested to as such, like the ten who were given glad tidings of Paradise, Bilal, Abdullah ibn Salam, the woman who used to have seizures, Ja'far ibn Abi Talib, and others.