I have read many related posts regarding the issue of having squared brackets with biblatex. Unfortunately, none have made me fully understand how to compute the squared brackets. I know that biblatex is not compatible with natbib, so that was not an option.
This is what it looks like now in the text:

This is what it looks like in the bibliography:

Disered output: I want to have squared brackets instead of the rounded brackets. It seems very simple, but after many frustrating hours I can't figure it out.
This is what it looks in code:
\usepackage[style=numeric, backend=biber]{biblatex} % Adjust bibliography style
\addbibresource{bibliography.bib} % BibTeX bibliography file
\DeclareFieldFormat{labelnumber}{\RN{#1}}
The MWE.tex file:
%----------------------------------------------------------------------------------------
% DOCUMENTCLASS
%----------------------------------------------------------------------------------------
\documentclass[11pt,a4paper,fleqn,twoside]{book} % Default font size and left-justified equations
%----------------------------------------------------------------------------------------
% FONTS
%----------------------------------------------------------------------------------------
% itemize with dash
\def\labelitemi{--}
\usepackage{avant} % Use the Avantgarde font
%
\renewcommand{\familydefault}{\sfdefault} % Makes changes in the text font
\usepackage{microtype} % Slightly tweak font spacing for aesthetics
\usepackage[utf8]{inputenc} % Required for including letters with accents
\usepackage[T1]{fontenc} % Use 8-bit encoding that has 256 glyphs
%----------------------------------------------------------------------------------------
% BIBLIOGRAPHY
%----------------------------------------------------------------------------------------
\usepackage{csquotes}
\usepackage[style=numeric, backend=biber]{biblatex} % Adjust bibliography style
\addbibresource{bibliography.bib} % BibTeX bibliography file
\DeclareFieldFormat{labelnumber}{\RN{#1}}
\defbibheading{bibempty}{}
%----------------------------------------------------------------------------------------
% START DOCUMENT
%----------------------------------------------------------------------------------------
\begin{document}
%----------------------------------------------------------------------------------------
% PART II
%----------------------------------------------------------------------------------------
Value-based health care is a health care delivery model that aims to achieve high value for patients, with value defined as health outcomes achieved per dollar spent \cite{porter2010value}.
%----------------------------------------------------------------------------------------
% BIBLIOGRAPHY
%----------------------------------------------------------------------------------------
\printbibliography
%----------------------------------------------------------------------------------------
% END DOCUMENT
%----------------------------------------------------------------------------------------
\end{document}
The bibliography.bib file:
@article{porter2010value,
title={What is value in health care?},
author={Porter, Michael E},
journal={New England Journal of Medicine},
volume={363},
number={26},
pages={2477--2481},
year={2010},
publisher={Mass Medical Soc}
}

biblatexwill produce square brackets withstyle=numeric,and nothing in the code shown in the question suggests otherwise. Can you please post a complete example document that reproduces the output you see. Please test the example document in a new, empty folder to ensure it really reproduces the issue. See https://tex.meta.stackexchange.com/q/228/35864 and https://tex.meta.stackexchange.com/q/4407/35864. – moewe Sep 29 '19 at 12:13biblatexissue at all. You may just have to choose a different font with more square square brackets. – moewe Sep 29 '19 at 12:14