Fi-Ne-Si-Te
The auxiliary function for INFP is extraverted intuition, otherwise known as Ne. Ne, being the extraverted function, serves to turn dominant Fi's attention towards the outside world. As a result from this, dominant Fi has the ability to understand other people more clearly, and can even discover shared values among a group. Auxiliary Ne also has the ability to increase creativity. When Fi is unhealthy, it restricts any new viewpoints and any info that disrupts their worldview. Letting Ne in decreases that stubbornness. However, if one uses Ne too much, they are prone to chasing multiple, often unrealistic ideas without restriction. INFP's must balance Fi and Ne appropriately so as to not become unhealthy.
The third function, the tertiary, here is Si. Tertiary Si makes its users take delight in reminiscing in the past, and can make them stubborn. Stubbornness seems to be the main side effect of Si in this position, as it has been noted that, when unhealthy, INFPs (and INTP also, which shares the auxiliary and tertiary functions) can become delusionally hold onto their personal values way past the point of realism.
Last, but not least, the inferior function for INFP's is extraverted thinking, Te. Inferior Te is the anthesis to dominant Fi. Te deals with concrete logic and objective reality, as I have noted before, and this has the ability to completely wreck Fi's inner world. Because of this, Te is repressed and is viewed as cold and overly critical. As you can guess, Fi is an individualist function and it dislikes generalizations made out of people, especially distrusting collectivism. What happens when Te goes repressed is we view life as subjective and misuse Te by using it to confirm our own biases. It can also cause rebelliousness when not properly wielded, and can cause us to also become too rigid, engaging in black-and-white morality. Fi-doms need to seek the true understanding of what Te entails and the benefits of it. One benefit that developing inferior Te provides is learning better critical thinking skills, this strengthens both Fi and Ne. A healthy Te strengthens all of the functions.
Fi-Se-Ni-Te
The auxiliary function for ISFP is extraverted sensation, Se. Like auxiliary Ne, Se helps turn dominant Fi towards the outside world. Se seeks to make Fi more adventurous and open-minded, and helps these users seek a balance between their personal world and the objective outside world. When Se is repressed, it makes ISFPs close-minded, much like what happens when auxiliary Ne is repressed. However, too much Se and one runs the risk of turning to dangerously risky behaviors and not considering how your actions will affect people. Long term, unhealthy Se leads to being disconnected and adopting possibly problematic ideals. A balance is needed between Fi and Se.
The tertiary function here is Ni. Since I already touched on inferior Te, this will be the last I will be describing. This position can give its users impressions about the future that can serve as a balance between the more concrete auxiliary Se. I once heard that tertiary Ni sometimes makes ISFPs believe in bizzare and illogical stuff. Both Ni and the tertiary position are hard to explain and also hard to find information about, so I apologize if this segment is short. Will update if I find something interesting.