Formal:
* Fidelity: This implies faithfulness and unwavering loyalty.
* Devoted: This emphasizes a strong sense of commitment and dedication.
* Unwavering: This suggests an unshakeable commitment, not swayed by circumstance.
* Eternal: This conveys a devotion that lasts forever.
* Indelible: This signifies a devotion that is deeply etched and cannot be erased.
Informal:
* True love: This emphasizes a deep, lasting connection.
* Soulmate: This suggests a perfect match and a profound connection.
* Lifelong: This conveys a devotion that lasts for the entirety of one's life.
* Unconditional: This emphasizes a devotion that is free of limitations or expectations.
* Heartfelt: This emphasizes the emotional depth of the devotion.
Ultimately, the best word to use will depend on the specific situation and the nuance you want to convey.