General synonyms:
* Comfortably
* Soothingly
* Sympathetically
* Compassionately
* Kindly
* Tenderly
* Reassuringly
* Encouragingly
More specific synonyms:
* Empathetically: if you want to emphasize understanding and sharing the other person's feelings.
* Warmly: if you want to emphasize affection and warmth.
* Patiently: if you want to emphasize taking the time to listen and offer support.
* Quietly: if you want to emphasize gentleness and calm.
The best synonym will depend on the specific context of your sentence.