I think many ISFJs have a low self-esteem. They are used give-in for others, and are constantly afraid of being seen as rude or selfish. They like to help others and have a need to be needed.
Therefore, make sure that the ISFJ has noticed that you have noticed him/her. Give them a chance to help you, and be sure to show that you couldn't have done it without their help. Someone referred ISFJs as "Sidekicks". That would mean that they are looking for someone they can help and support.
But generally ISFJs are somewhat reserved in these matters.
Use my advices at your own risk.
(Oh, and if I sounded harsh towards ISFJs, I didn't mean to be harsh. They are just the kind of people you always find helping and looking after for you.

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