I have a helper like
Template.user_profile.helpers({
  user:function() {
     return Meteor.users.find({\'profile.front_name\':Session.get(\'slug\')}).fetch(         
        
You can call a template helper (not global helper - which is in outluch's answer) with:
Template.tplName.__helpers.get('helper').call()
MDG suggests using a regular function and then passing it to helpers, events and so on. See here.
Update 16.06.16
Actually I strongly advise to simply use manuel:viewmodel - it alleviates so many Blaze headaches...