General synonyms:
* Permanent
* Lasting
* Everlasting
* Unforgettable
* Unerasable
* Imprinted
* Engraved
* Inherent
* Intrinsic
More specific synonyms:
* For memories: unforgettable, etched, ingrained
* For marks: permanent, enduring, fixed, unfading
* For impressions: profound, deep-seated, ingrained
The best synonym to use will depend on the specific situation and the meaning you want to convey.