The difference of systematic review compared to narrative review is to ensure the objectivity and reproducibility. The searching process should be organized with pre-established inclusion and exclusion criteria and evaluating the methodological quality of included studies should be used with specific evaluation tools. All the process should be done with more than 2 researchers relevant to the research area and the disagreements or discrepancy of review process between researchers should be resolved by discussion or referring to another specialists. After finishing all the process stated above, review of literature follows and researchers decide whether to perform meta-analysis or descriptive review without meta-analysis according to the homogeneity of included data. Descriptive review process usually does not have an element for critical analysis. However, researchers can adopt critical analysis if needed.

If you mean the introduction, that should be descriptive regardless of whether it is a LR or a SLR. It should describe the problem, previous research/ other similar reviews, and explain what your review is adding. Hope this helps.