The pair were bandmates alongside Harry Styles, Liam Payne and Zayn Malik when they first started out, and the 23-year-old singer has praised the father-of-one for helping the group remain level-headed during turbulent times.
He said: ''He's an unbelievable guy, a great singer and someone that if, I'd say, we would have lost the plot along the way if we hadn't had Louis.''
Niall also insisted Louis is an ''extremely vital'' One Direction member after the 25-year-old star recently revealed he suffered from insecurity as he tried to find his place in the group.
Speaking on Australian radio station Nova 100, Niall said: ''He was a very, very, very, extremely vital member of One Direction. He's a great man.
''I thought it was very honest interview, I thought he was brilliant in it. He knows that none of us ever thought that or think that.''
Louis recently opened up about his friendship with his mother Johannah Deakin - who died of leukaemia last December - and admitted he is thankful that she was always supportive.
He said: ''I'm not gonna claim this is all for me mum. But it was definitely ... It was.
''Sometimes my reservation, or my confidence, might have prevented me from doing something. And I've needed a mum in the past to kick me up the arse and go, 'You're doing it.' ''
The star - who has 18-month-old son Freddie - admitted she was the first person he confided in after he lost his virginity.
Louis explained: ''I remember the day I lost my virginity. I hadn't even told any of my mates and I was, like, 'Mum? I know this is really weird. But I've got to tell you...' I remember thinking this is a bizarre conversation to be having with your mother.
''But it's testament to how comfortable she made me.''
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